target/riscv/cpu.c | 25 ++++++++++ target/riscv/cpu.h | 10 ++++ target/riscv/cpu_cfg.h | 2 + target/riscv/kvm/kvm-cpu.c | 5 +- target/riscv/tcg/tcg-cpu.c | 98 ++++++++++++++++++++++++++++++++++++++ 5 files changed, 139 insertions(+), 1 deletion(-)
Based-on: 20230926183109.165878-1-dbarboza@ventanamicro.com
("[PATCH 0/2] riscv: add extension properties for all cpus")
Hi,
These patches implements the base profile support for qemu-riscv and the
first profile, RVA22U64.
As discussed in this thread [1] we're aiming for a flag that enables all
mandatory extensions of a profile. Optional extensions were left behind
and must be enabled by hand if desired. Since this is the first profile
we're adding, we'll need to add the base framework as well.
The RVA22U64 profile was chosen because qemu-riscv implements all its
extensions, both mandatory and optional. That includes 'zicntr' and
'zihpm', which we support for awhile but aren't adverting to userspace.
Other design decisions made:
- disabling a profile flag does nothing, i.e. we won't mass disable
mandatory extensions of the rva22U64 profile if the user sets
rva22u64=false;
- profile support for vendor CPUs consists into checking if the CPU
happens to have the mandatory extensions required for it. In case it
doesn't we'll error out. This is done to follow the same prerogative
we always had of not allowing extensions being enabled for vendor
CPUs;
- the KVM driver doesn't support profiles. In theory we could apply the
same logic as for the vendor CPUs, but KVM has a long way to go before
that becomes a factor. We'll revisit this decision when KVM is able to
support at least one profile.
Patch 5 ("enable profile support for vendor CPUs") needs the following
series to be applied beforehand:
"[PATCH 0/2] riscv: add extension properties for all cpus"
Otherwise we won't be able to add the profile flag to vendor CPUs.
[1] https://lore.kernel.org/qemu-riscv/35a847a1-2720-14ab-61b0-c72d77d5f43b@ventanamicro.com/
Daniel Henrique Barboza (6):
target/riscv/cpu.c: add zicntr extension flag
target/riscv/cpu.c: add zihpm extension flag
target/riscv: add rva22u64 profile definition
target/riscv/tcg: implement rva22u64 profile
target/riscv/tcg-cpu.c: enable profile support for vendor CPUs
target/riscv/kvm: add 'rva22u64' flag as unavailable
target/riscv/cpu.c | 25 ++++++++++
target/riscv/cpu.h | 10 ++++
target/riscv/cpu_cfg.h | 2 +
target/riscv/kvm/kvm-cpu.c | 5 +-
target/riscv/tcg/tcg-cpu.c | 98 ++++++++++++++++++++++++++++++++++++++
5 files changed, 139 insertions(+), 1 deletion(-)
--
2.41.0
On Tue, Sep 26, 2023 at 04:49:44PM -0300, Daniel Henrique Barboza wrote:
> Based-on: 20230926183109.165878-1-dbarboza@ventanamicro.com
> ("[PATCH 0/2] riscv: add extension properties for all cpus")
>
> Hi,
>
> These patches implements the base profile support for qemu-riscv and the
> first profile, RVA22U64.
>
> As discussed in this thread [1] we're aiming for a flag that enables all
> mandatory extensions of a profile. Optional extensions were left behind
> and must be enabled by hand if desired. Since this is the first profile
> we're adding, we'll need to add the base framework as well.
>
> The RVA22U64 profile was chosen because qemu-riscv implements all its
> extensions, both mandatory and optional. That includes 'zicntr' and
> 'zihpm', which we support for awhile but aren't adverting to userspace.
>
> Other design decisions made:
>
> - disabling a profile flag does nothing, i.e. we won't mass disable
> mandatory extensions of the rva22U64 profile if the user sets
> rva22u64=false;
Wouldn't it make more sense to error out when this is requested?
Silently ignoring an explicit request made by the user is pretty much
never a good idea in my experience.
--
Andrea Bolognani / Red Hat / Virtualization
On Tue, Sep 26, 2023 at 04:49:44PM -0300, Daniel Henrique Barboza wrote:
> Based-on: 20230926183109.165878-1-dbarboza@ventanamicro.com
> ("[PATCH 0/2] riscv: add extension properties for all cpus")
>
> Hi,
>
> These patches implements the base profile support for qemu-riscv and the
> first profile, RVA22U64.
>
> As discussed in this thread [1] we're aiming for a flag that enables all
> mandatory extensions of a profile. Optional extensions were left behind
> and must be enabled by hand if desired. Since this is the first profile
> we're adding, we'll need to add the base framework as well.
>
> The RVA22U64 profile was chosen because qemu-riscv implements all its
> extensions, both mandatory and optional. That includes 'zicntr' and
> 'zihpm', which we support for awhile but aren't adverting to userspace.
>
> Other design decisions made:
>
> - disabling a profile flag does nothing, i.e. we won't mass disable
> mandatory extensions of the rva22U64 profile if the user sets
> rva22u64=false;
Why shouldn't this be allowed ?
IIUC, a profile is syntactic sugar for a group of features. If
we can disable individual features explicitly, why should we
not allow use of the profile as sugar to disable them en-mass ?
BTW, I would caution that the semantics of mixing groups of
features, with individual features in -cpu is likely to be
ill defined, as you cannot rely on left-to-right processing
of the -cpu arguments.
IOW, if you say -cpu $foo,$group=on,$feature=off
you might expect this to result in '$feature' being disabled
if it were implied by $group. This is not guaranteed as the
QDict processing of options could result in us effectively
processing -cpu $foo,$feature=off,$group=on
This brokeness with CPU feature groups and their interaction
with CPU feature flags already impacts s390x:
https://lists.nongnu.org/archive/html/qemu-devel/2023-09/msg00981.html
There is a possible way to fix it by declaring an ordering
such that all groups will be processed fully, before any
individual features are processed, and declaring that group
or feature names must not be repeated. This avoids a reliance
on left-to-right ordering:
https://lists.nongnu.org/archive/html/qemu-devel/2023-09/msg01005.html
that is still likely broken, however, if multiple different
groups are listed, and there is overlap in their feature
sets.
TL;DR: feature groups are pretty error prone if more than
one is listed by the user, or they're combined with individual
features.
>
> - profile support for vendor CPUs consists into checking if the CPU
> happens to have the mandatory extensions required for it. In case it
> doesn't we'll error out. This is done to follow the same prerogative
> we always had of not allowing extensions being enabled for vendor
> CPUs;
Why shouldn't this be allowed ?
> - the KVM driver doesn't support profiles. In theory we could apply the
> same logic as for the vendor CPUs, but KVM has a long way to go before
> that becomes a factor. We'll revisit this decision when KVM is able to
> support at least one profile.
>
> Patch 5 ("enable profile support for vendor CPUs") needs the following
> series to be applied beforehand:
>
> "[PATCH 0/2] riscv: add extension properties for all cpus"
>
> Otherwise we won't be able to add the profile flag to vendor CPUs.
>
> [1] https://lore.kernel.org/qemu-riscv/35a847a1-2720-14ab-61b0-c72d77d5f43b@ventanamicro.com/
>
> Daniel Henrique Barboza (6):
> target/riscv/cpu.c: add zicntr extension flag
> target/riscv/cpu.c: add zihpm extension flag
> target/riscv: add rva22u64 profile definition
> target/riscv/tcg: implement rva22u64 profile
> target/riscv/tcg-cpu.c: enable profile support for vendor CPUs
> target/riscv/kvm: add 'rva22u64' flag as unavailable
>
> target/riscv/cpu.c | 25 ++++++++++
> target/riscv/cpu.h | 10 ++++
> target/riscv/cpu_cfg.h | 2 +
> target/riscv/kvm/kvm-cpu.c | 5 +-
> target/riscv/tcg/tcg-cpu.c | 98 ++++++++++++++++++++++++++++++++++++++
> 5 files changed, 139 insertions(+), 1 deletion(-)
>
> --
> 2.41.0
>
>
With regards,
Daniel
--
|: https://berrange.com -o- https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org -o- https://fstop138.berrange.com :|
|: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
On 9/29/23 07:46, Daniel P. Berrangé wrote:
> On Tue, Sep 26, 2023 at 04:49:44PM -0300, Daniel Henrique Barboza wrote:
>> Based-on: 20230926183109.165878-1-dbarboza@ventanamicro.com
>> ("[PATCH 0/2] riscv: add extension properties for all cpus")
>>
>> Hi,
>>
>> These patches implements the base profile support for qemu-riscv and the
>> first profile, RVA22U64.
>>
>> As discussed in this thread [1] we're aiming for a flag that enables all
>> mandatory extensions of a profile. Optional extensions were left behind
>> and must be enabled by hand if desired. Since this is the first profile
>> we're adding, we'll need to add the base framework as well.
>>
>> The RVA22U64 profile was chosen because qemu-riscv implements all its
>> extensions, both mandatory and optional. That includes 'zicntr' and
>> 'zihpm', which we support for awhile but aren't adverting to userspace.
>>
>> Other design decisions made:
>>
>> - disabling a profile flag does nothing, i.e. we won't mass disable
>> mandatory extensions of the rva22U64 profile if the user sets
>> rva22u64=false;
>
> Why shouldn't this be allowed ?
>
> IIUC, a profile is syntactic sugar for a group of features. If
> we can disable individual features explicitly, why should we
> not allow use of the profile as sugar to disable them en-mass ?
In theory there's no harm in allowing mass disabling of extensions but, given
it's a whole profile, we would end up disabling most/all CPU extensions and
the guest would do nothing.
There is a thread in the ML:
https://lore.kernel.org/qemu-riscv/CABJz62NyVNu4Z1qmCG7MyJkGG_9yWxjUFHHWjmoQEP6unRrHNA@mail.gmail.com/
Where we discussed the possibility of having a minimal CPU extension set. We didn't
reach a consensus because the definition of "minimal CPU extension set" vary between
OSes (Linux requires IMAFD, FreeBSD might require something differ).
Assuming we reach a consensus on what a minimal set is, we could allow disabling mass
extensions via probile but keeping this minimal set, for example. At very least we
shouldn't allow users to disable 'I' because that would kill the CPU, so RV64I is
the minimum set that I would assume for now.
>
>
> BTW, I would caution that the semantics of mixing groups of
> features, with individual features in -cpu is likely to be
> ill defined, as you cannot rely on left-to-right processing
> of the -cpu arguments.
>
> IOW, if you say -cpu $foo,$group=on,$feature=off
>
> you might expect this to result in '$feature' being disabled
> if it were implied by $group. This is not guaranteed as the
> QDict processing of options could result in us effectively
> processing -cpu $foo,$feature=off,$group=on
>
> This brokeness with CPU feature groups and their interaction
> with CPU feature flags already impacts s390x:
>
> https://lists.nongnu.org/archive/html/qemu-devel/2023-09/msg00981.html
>
> There is a possible way to fix it by declaring an ordering
> such that all groups will be processed fully, before any
> individual features are processed, and declaring that group
> or feature names must not be repeated. This avoids a reliance
> on left-to-right ordering:
>
> https://lists.nongnu.org/archive/html/qemu-devel/2023-09/msg01005.html
>
> that is still likely broken, however, if multiple different
> groups are listed, and there is overlap in their feature
> sets.
Just read the discussion. I agree with restricting the command line flexibility
to make things more consistent, but IIRC libvirt (and others) uses a lot of
command line appending and restricting these use cases now will break stuff
up. I recall pc64 domains using <qemu:commandline> to add extra cpu/machine
flags to the domain back in the day. Surely we can claim (I guess) that command
line pass-through is unstable and users shouldn't expect extended support for
it, but we all know that users will be less than pleased when their domains
start breaking because QEMU restricted the command line.
As for the current RISC-V case, one thing we can do is to postpone feature group
processing to realize() time, since in that stage we're already done with the
command line processing. We can make a sanity check between the feature group
flags and error out if there's something off. That would make things a little
less fragile that what they are, albeit I'm sure that there will be some cases
that will be left uncovered.
>
> TL;DR: feature groups are pretty error prone if more than
> one is listed by the user, or they're combined with individual
> features.
>
>>
>> - profile support for vendor CPUs consists into checking if the CPU
>> happens to have the mandatory extensions required for it. In case it
>> doesn't we'll error out. This is done to follow the same prerogative
>> we always had of not allowing extensions being enabled for vendor
>> CPUs;
>
> Why shouldn't this be allowed ?
There's no technical reason to not allow it. The reason it's forbid is to be
closer to what the real hardware would do. E.g. the real hardware doesn't allow
users to enable Vector if the hardware doesn't support it. Vendor CPUs also has
a privileged spec restriction as well, so if a CPU is running in an older spec
it can't enable extensions that were added later.
If vendor CPUs from x86 and others behave in a different way with feature enablement
I'd like to hear about it. I can say that what RISC-V is doing in this regard is
not that far from what PowerPC does.
Thanks,
Daniel
>
>> - the KVM driver doesn't support profiles. In theory we could apply the
>> same logic as for the vendor CPUs, but KVM has a long way to go before
>> that becomes a factor. We'll revisit this decision when KVM is able to
>> support at least one profile.
>>
>> Patch 5 ("enable profile support for vendor CPUs") needs the following
>> series to be applied beforehand:
>>
>> "[PATCH 0/2] riscv: add extension properties for all cpus"
>>
>> Otherwise we won't be able to add the profile flag to vendor CPUs.
>>
>> [1] https://lore.kernel.org/qemu-riscv/35a847a1-2720-14ab-61b0-c72d77d5f43b@ventanamicro.com/
>>
>> Daniel Henrique Barboza (6):
>> target/riscv/cpu.c: add zicntr extension flag
>> target/riscv/cpu.c: add zihpm extension flag
>> target/riscv: add rva22u64 profile definition
>> target/riscv/tcg: implement rva22u64 profile
>> target/riscv/tcg-cpu.c: enable profile support for vendor CPUs
>> target/riscv/kvm: add 'rva22u64' flag as unavailable
>>
>> target/riscv/cpu.c | 25 ++++++++++
>> target/riscv/cpu.h | 10 ++++
>> target/riscv/cpu_cfg.h | 2 +
>> target/riscv/kvm/kvm-cpu.c | 5 +-
>> target/riscv/tcg/tcg-cpu.c | 98 ++++++++++++++++++++++++++++++++++++++
>> 5 files changed, 139 insertions(+), 1 deletion(-)
>>
>> --
>> 2.41.0
>>
>>
>
> With regards,
> Daniel
On Fri, Sep 29, 2023 at 08:29:08AM -0300, Daniel Henrique Barboza wrote:
>
>
> On 9/29/23 07:46, Daniel P. Berrangé wrote:
> > On Tue, Sep 26, 2023 at 04:49:44PM -0300, Daniel Henrique Barboza wrote:
> > > Based-on: 20230926183109.165878-1-dbarboza@ventanamicro.com
> > > ("[PATCH 0/2] riscv: add extension properties for all cpus")
> > >
> > > Hi,
> > >
> > > These patches implements the base profile support for qemu-riscv and the
> > > first profile, RVA22U64.
> > >
> > > As discussed in this thread [1] we're aiming for a flag that enables all
> > > mandatory extensions of a profile. Optional extensions were left behind
> > > and must be enabled by hand if desired. Since this is the first profile
> > > we're adding, we'll need to add the base framework as well.
> > >
> > > The RVA22U64 profile was chosen because qemu-riscv implements all its
> > > extensions, both mandatory and optional. That includes 'zicntr' and
> > > 'zihpm', which we support for awhile but aren't adverting to userspace.
> > >
> > > Other design decisions made:
> > >
> > > - disabling a profile flag does nothing, i.e. we won't mass disable
> > > mandatory extensions of the rva22U64 profile if the user sets
> > > rva22u64=false;
> >
> > Why shouldn't this be allowed ?
> >
> > IIUC, a profile is syntactic sugar for a group of features. If
> > we can disable individual features explicitly, why should we
> > not allow use of the profile as sugar to disable them en-mass ?
>
> In theory there's no harm in allowing mass disabling of extensions but, given
> it's a whole profile, we would end up disabling most/all CPU extensions and
> the guest would do nothing.
True, that is just user error though. They could disable a profile
and then manually re-enable individual features, and thus get a
working system.
> There is a thread in the ML:
>
> https://lore.kernel.org/qemu-riscv/CABJz62NyVNu4Z1qmCG7MyJkGG_9yWxjUFHHWjmoQEP6unRrHNA@mail.gmail.com/
>
> Where we discussed the possibility of having a minimal CPU extension set. We didn't
> reach a consensus because the definition of "minimal CPU extension set" vary between
> OSes (Linux requires IMAFD, FreeBSD might require something differ).
>
> Assuming we reach a consensus on what a minimal set is, we could allow disabling mass
> extensions via probile but keeping this minimal set, for example. At very least we
> shouldn't allow users to disable 'I' because that would kill the CPU, so RV64I is
> the minimum set that I would assume for now.
I'd probably just call that user error too.
> >
> > TL;DR: feature groups are pretty error prone if more than
> > one is listed by the user, or they're combined with individual
> > features.
> >
> > >
> > > - profile support for vendor CPUs consists into checking if the CPU
> > > happens to have the mandatory extensions required for it. In case it
> > > doesn't we'll error out. This is done to follow the same prerogative
> > > we always had of not allowing extensions being enabled for vendor
> > > CPUs;
> >
> > Why shouldn't this be allowed ?
>
> There's no technical reason to not allow it. The reason it's forbid is to be
> closer to what the real hardware would do. E.g. the real hardware doesn't allow
> users to enable Vector if the hardware doesn't support it. Vendor CPUs also has
> a privileged spec restriction as well, so if a CPU is running in an older spec
> it can't enable extensions that were added later.
Real hardware is constrained in not being able to invent arbitrary
new features on chip. Virtual machines are not constrained, so
I don't think the inability of hardware todo this, is an especially
strong reason to limit software emulation.
What I don't like about this, is that (IIUC) the '$profile=on' option
now has different semantics depending on what CPU it is used with.
ie using it with a vendor CPU, $profile=on becomes an assertion
that the vendor CPU contains all the features needed to satisfy
$profile. It won't enable/disable anything, just check it is present.
With a non-vendor CPU, using $profile=on becomes a mechanism to force
enable all the features needed to satisfy $profile, there is no
mechanism to just check for presence.
Having two different semantics for the same syntax is generally considered
bad design practice.
This points towards supporting a tri-state, not boolean. $profile=check
for validation only, and $profile=on for force enablement.
With regards,
Daniel
--
|: https://berrange.com -o- https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org -o- https://fstop138.berrange.com :|
|: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
On 9/29/23 08:55, Daniel P. Berrangé wrote:
> On Fri, Sep 29, 2023 at 08:29:08AM -0300, Daniel Henrique Barboza wrote:
>>
>>
>> On 9/29/23 07:46, Daniel P. Berrangé wrote:
>>> On Tue, Sep 26, 2023 at 04:49:44PM -0300, Daniel Henrique Barboza wrote:
>>>> Based-on: 20230926183109.165878-1-dbarboza@ventanamicro.com
>>>> ("[PATCH 0/2] riscv: add extension properties for all cpus")
>>>>
>>>> Hi,
>>>>
>>>> These patches implements the base profile support for qemu-riscv and the
>>>> first profile, RVA22U64.
>>>>
>>>> As discussed in this thread [1] we're aiming for a flag that enables all
>>>> mandatory extensions of a profile. Optional extensions were left behind
>>>> and must be enabled by hand if desired. Since this is the first profile
>>>> we're adding, we'll need to add the base framework as well.
>>>>
>>>> The RVA22U64 profile was chosen because qemu-riscv implements all its
>>>> extensions, both mandatory and optional. That includes 'zicntr' and
>>>> 'zihpm', which we support for awhile but aren't adverting to userspace.
>>>>
>>>> Other design decisions made:
>>>>
>>>> - disabling a profile flag does nothing, i.e. we won't mass disable
>>>> mandatory extensions of the rva22U64 profile if the user sets
>>>> rva22u64=false;
>>>
>>> Why shouldn't this be allowed ?
>>>
>>> IIUC, a profile is syntactic sugar for a group of features. If
>>> we can disable individual features explicitly, why should we
>>> not allow use of the profile as sugar to disable them en-mass ?
>>
>> In theory there's no harm in allowing mass disabling of extensions but, given
>> it's a whole profile, we would end up disabling most/all CPU extensions and
>> the guest would do nothing.
>
> True, that is just user error though. They could disable a profile
> and then manually re-enable individual features, and thus get a
> working system.
>
>> There is a thread in the ML:
>>
>> https://lore.kernel.org/qemu-riscv/CABJz62NyVNu4Z1qmCG7MyJkGG_9yWxjUFHHWjmoQEP6unRrHNA@mail.gmail.com/
>>
>> Where we discussed the possibility of having a minimal CPU extension set. We didn't
>> reach a consensus because the definition of "minimal CPU extension set" vary between
>> OSes (Linux requires IMAFD, FreeBSD might require something differ).
>>
>> Assuming we reach a consensus on what a minimal set is, we could allow disabling mass
>> extensions via probile but keeping this minimal set, for example. At very least we
>> shouldn't allow users to disable 'I' because that would kill the CPU, so RV64I is
>> the minimum set that I would assume for now.
>
> I'd probably just call that user error too.
>
>>>
>>> TL;DR: feature groups are pretty error prone if more than
>>> one is listed by the user, or they're combined with individual
>>> features.
>>>
>>>>
>>>> - profile support for vendor CPUs consists into checking if the CPU
>>>> happens to have the mandatory extensions required for it. In case it
>>>> doesn't we'll error out. This is done to follow the same prerogative
>>>> we always had of not allowing extensions being enabled for vendor
>>>> CPUs;
>>>
>>> Why shouldn't this be allowed ?
>>
>> There's no technical reason to not allow it. The reason it's forbid is to be
>> closer to what the real hardware would do. E.g. the real hardware doesn't allow
>> users to enable Vector if the hardware doesn't support it. Vendor CPUs also has
>> a privileged spec restriction as well, so if a CPU is running in an older spec
>> it can't enable extensions that were added later.
>
> Real hardware is constrained in not being able to invent arbitrary
> new features on chip. Virtual machines are not constrained, so
> I don't think the inability of hardware todo this, is an especially
> strong reason to limit software emulation.
>
> What I don't like about this, is that (IIUC) the '$profile=on' option
> now has different semantics depending on what CPU it is used with.
>
> ie using it with a vendor CPU, $profile=on becomes an assertion
> that the vendor CPU contains all the features needed to satisfy
> $profile. It won't enable/disable anything, just check it is present.
>
> With a non-vendor CPU, using $profile=on becomes a mechanism to force
> enable all the features needed to satisfy $profile, there is no
> mechanism to just check for presence.
>
> Having two different semantics for the same syntax is generally considered
> bad design practice.
>
> This points towards supporting a tri-state, not boolean. $profile=check
> for validation only, and $profile=on for force enablement.
This would leave us with:
- $profile=off => disable all extensions. Let users hit themselves in the foot if they
don't enable any other extensions. Note that disabling a profile and enabling extensions
on top of it is very sensitive to left-to-right ordering, so it would be good to have
a way to enforce this ordering somehow (feature groups always first);
- $profile=on => only valid for generic CPUs;
- $profile=check -> valid for all CPUs, would only check if the CPU implements the profile.
I think this is fine. Drew, care to weight in?
Thanks,
Daniel
>
>
> With regards,
> Daniel
On Fri, Sep 29, 2023 at 09:49:47AM -0300, Daniel Henrique Barboza wrote:
>
>
> On 9/29/23 08:55, Daniel P. Berrangé wrote:
> > On Fri, Sep 29, 2023 at 08:29:08AM -0300, Daniel Henrique Barboza wrote:
> > >
> > >
> > > On 9/29/23 07:46, Daniel P. Berrangé wrote:
> > > > On Tue, Sep 26, 2023 at 04:49:44PM -0300, Daniel Henrique Barboza wrote:
> > > > > Based-on: 20230926183109.165878-1-dbarboza@ventanamicro.com
> > > > > ("[PATCH 0/2] riscv: add extension properties for all cpus")
> > > > >
> > > > > Hi,
> > > > >
> > > > > These patches implements the base profile support for qemu-riscv and the
> > > > > first profile, RVA22U64.
> > > > >
> > > > > As discussed in this thread [1] we're aiming for a flag that enables all
> > > > > mandatory extensions of a profile. Optional extensions were left behind
> > > > > and must be enabled by hand if desired. Since this is the first profile
> > > > > we're adding, we'll need to add the base framework as well.
> > > > >
> > > > > The RVA22U64 profile was chosen because qemu-riscv implements all its
> > > > > extensions, both mandatory and optional. That includes 'zicntr' and
> > > > > 'zihpm', which we support for awhile but aren't adverting to userspace.
> > > > >
> > > > > Other design decisions made:
> > > > >
> > > > > - disabling a profile flag does nothing, i.e. we won't mass disable
> > > > > mandatory extensions of the rva22U64 profile if the user sets
> > > > > rva22u64=false;
> > > >
> > > > Why shouldn't this be allowed ?
> > > >
> > > > IIUC, a profile is syntactic sugar for a group of features. If
> > > > we can disable individual features explicitly, why should we
> > > > not allow use of the profile as sugar to disable them en-mass ?
> > >
> > > In theory there's no harm in allowing mass disabling of extensions but, given
> > > it's a whole profile, we would end up disabling most/all CPU extensions and
> > > the guest would do nothing.
> >
> > True, that is just user error though. They could disable a profile
> > and then manually re-enable individual features, and thus get a
> > working system.
> >
> > > There is a thread in the ML:
> > >
> > > https://lore.kernel.org/qemu-riscv/CABJz62NyVNu4Z1qmCG7MyJkGG_9yWxjUFHHWjmoQEP6unRrHNA@mail.gmail.com/
> > >
> > > Where we discussed the possibility of having a minimal CPU extension set. We didn't
> > > reach a consensus because the definition of "minimal CPU extension set" vary between
> > > OSes (Linux requires IMAFD, FreeBSD might require something differ).
> > >
> > > Assuming we reach a consensus on what a minimal set is, we could allow disabling mass
> > > extensions via probile but keeping this minimal set, for example. At very least we
> > > shouldn't allow users to disable 'I' because that would kill the CPU, so RV64I is
> > > the minimum set that I would assume for now.
> >
> > I'd probably just call that user error too.
> >
> > > >
> > > > TL;DR: feature groups are pretty error prone if more than
> > > > one is listed by the user, or they're combined with individual
> > > > features.
> > > >
> > > > >
> > > > > - profile support for vendor CPUs consists into checking if the CPU
> > > > > happens to have the mandatory extensions required for it. In case it
> > > > > doesn't we'll error out. This is done to follow the same prerogative
> > > > > we always had of not allowing extensions being enabled for vendor
> > > > > CPUs;
> > > >
> > > > Why shouldn't this be allowed ?
> > >
> > > There's no technical reason to not allow it. The reason it's forbid is to be
> > > closer to what the real hardware would do. E.g. the real hardware doesn't allow
> > > users to enable Vector if the hardware doesn't support it. Vendor CPUs also has
> > > a privileged spec restriction as well, so if a CPU is running in an older spec
> > > it can't enable extensions that were added later.
> >
> > Real hardware is constrained in not being able to invent arbitrary
> > new features on chip. Virtual machines are not constrained, so
> > I don't think the inability of hardware todo this, is an especially
> > strong reason to limit software emulation.
> >
> > What I don't like about this, is that (IIUC) the '$profile=on' option
> > now has different semantics depending on what CPU it is used with.
> >
> > ie using it with a vendor CPU, $profile=on becomes an assertion
> > that the vendor CPU contains all the features needed to satisfy
> > $profile. It won't enable/disable anything, just check it is present.
> >
> > With a non-vendor CPU, using $profile=on becomes a mechanism to force
> > enable all the features needed to satisfy $profile, there is no
> > mechanism to just check for presence.
> >
> > Having two different semantics for the same syntax is generally considered
> > bad design practice.
> >
> > This points towards supporting a tri-state, not boolean. $profile=check
> > for validation only, and $profile=on for force enablement.
>
> This would leave us with:
>
> - $profile=off => disable all extensions. Let users hit themselves in the foot if they
> don't enable any other extensions. Note that disabling a profile and enabling extensions
> on top of it is very sensitive to left-to-right ordering, so it would be good to have
> a way to enforce this ordering somehow (feature groups always first);
It is also order sensitive if 2 profiles have overlap in the
extensions they represent. So might also require an ordering
of profiles themselves to be defined if you permit multiple
profiles.
If we dont want to think about this immediately that, then
we should make $profile=off into a fatal error rather than
silently ignoring it
> - $profile=on => only valid for generic CPUs;
>
> - $profile=check -> valid for all CPUs, would only check if the CPU implements the profile.
>
>
> I think this is fine. Drew, care to weight in?
With regards,
Daniel
--
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On Fri, Sep 29, 2023 at 10:54 PM Daniel P. Berrangé <berrange@redhat.com> wrote:
>
> On Fri, Sep 29, 2023 at 09:49:47AM -0300, Daniel Henrique Barboza wrote:
> >
> >
> > On 9/29/23 08:55, Daniel P. Berrangé wrote:
> > > On Fri, Sep 29, 2023 at 08:29:08AM -0300, Daniel Henrique Barboza wrote:
> > > >
> > > >
> > > > On 9/29/23 07:46, Daniel P. Berrangé wrote:
> > > > > On Tue, Sep 26, 2023 at 04:49:44PM -0300, Daniel Henrique Barboza wrote:
> > > > > > Based-on: 20230926183109.165878-1-dbarboza@ventanamicro.com
> > > > > > ("[PATCH 0/2] riscv: add extension properties for all cpus")
> > > > > >
> > > > > > Hi,
> > > > > >
> > > > > > These patches implements the base profile support for qemu-riscv and the
> > > > > > first profile, RVA22U64.
> > > > > >
> > > > > > As discussed in this thread [1] we're aiming for a flag that enables all
> > > > > > mandatory extensions of a profile. Optional extensions were left behind
> > > > > > and must be enabled by hand if desired. Since this is the first profile
> > > > > > we're adding, we'll need to add the base framework as well.
> > > > > >
> > > > > > The RVA22U64 profile was chosen because qemu-riscv implements all its
> > > > > > extensions, both mandatory and optional. That includes 'zicntr' and
> > > > > > 'zihpm', which we support for awhile but aren't adverting to userspace.
> > > > > >
> > > > > > Other design decisions made:
> > > > > >
> > > > > > - disabling a profile flag does nothing, i.e. we won't mass disable
> > > > > > mandatory extensions of the rva22U64 profile if the user sets
> > > > > > rva22u64=false;
> > > > >
> > > > > Why shouldn't this be allowed ?
> > > > >
> > > > > IIUC, a profile is syntactic sugar for a group of features. If
> > > > > we can disable individual features explicitly, why should we
> > > > > not allow use of the profile as sugar to disable them en-mass ?
> > > >
> > > > In theory there's no harm in allowing mass disabling of extensions but, given
> > > > it's a whole profile, we would end up disabling most/all CPU extensions and
> > > > the guest would do nothing.
> > >
> > > True, that is just user error though. They could disable a profile
> > > and then manually re-enable individual features, and thus get a
> > > working system.
> > >
> > > > There is a thread in the ML:
> > > >
> > > > https://lore.kernel.org/qemu-riscv/CABJz62NyVNu4Z1qmCG7MyJkGG_9yWxjUFHHWjmoQEP6unRrHNA@mail.gmail.com/
> > > >
> > > > Where we discussed the possibility of having a minimal CPU extension set. We didn't
> > > > reach a consensus because the definition of "minimal CPU extension set" vary between
> > > > OSes (Linux requires IMAFD, FreeBSD might require something differ).
> > > >
> > > > Assuming we reach a consensus on what a minimal set is, we could allow disabling mass
> > > > extensions via probile but keeping this minimal set, for example. At very least we
> > > > shouldn't allow users to disable 'I' because that would kill the CPU, so RV64I is
> > > > the minimum set that I would assume for now.
> > >
> > > I'd probably just call that user error too.
> > >
> > > > >
> > > > > TL;DR: feature groups are pretty error prone if more than
> > > > > one is listed by the user, or they're combined with individual
> > > > > features.
> > > > >
> > > > > >
> > > > > > - profile support for vendor CPUs consists into checking if the CPU
> > > > > > happens to have the mandatory extensions required for it. In case it
> > > > > > doesn't we'll error out. This is done to follow the same prerogative
> > > > > > we always had of not allowing extensions being enabled for vendor
> > > > > > CPUs;
> > > > >
> > > > > Why shouldn't this be allowed ?
> > > >
> > > > There's no technical reason to not allow it. The reason it's forbid is to be
> > > > closer to what the real hardware would do. E.g. the real hardware doesn't allow
> > > > users to enable Vector if the hardware doesn't support it. Vendor CPUs also has
> > > > a privileged spec restriction as well, so if a CPU is running in an older spec
> > > > it can't enable extensions that were added later.
> > >
> > > Real hardware is constrained in not being able to invent arbitrary
> > > new features on chip. Virtual machines are not constrained, so
> > > I don't think the inability of hardware todo this, is an especially
> > > strong reason to limit software emulation.
I think exposing flexibility in vendor CPUs just creates confusion.
As a user if I start QEMU with "-cpu company-cpu" then I am expecting
to get an emulation of company-cpu.
> > >
> > > What I don't like about this, is that (IIUC) the '$profile=on' option
> > > now has different semantics depending on what CPU it is used with.
> > >
> > > ie using it with a vendor CPU, $profile=on becomes an assertion
> > > that the vendor CPU contains all the features needed to satisfy
> > > $profile. It won't enable/disable anything, just check it is present.
> > >
> > > With a non-vendor CPU, using $profile=on becomes a mechanism to force
> > > enable all the features needed to satisfy $profile, there is no
> > > mechanism to just check for presence.
> > >
> > > Having two different semantics for the same syntax is generally considered
> > > bad design practice.
> > >
> > > This points towards supporting a tri-state, not boolean. $profile=check
> > > for validation only, and $profile=on for force enablement.
> >
> > This would leave us with:
> >
> > - $profile=off => disable all extensions. Let users hit themselves in the foot if they
> > don't enable any other extensions. Note that disabling a profile and enabling extensions
> > on top of it is very sensitive to left-to-right ordering, so it would be good to have
> > a way to enforce this ordering somehow (feature groups always first);
>
> It is also order sensitive if 2 profiles have overlap in the
> extensions they represent. So might also require an ordering
> of profiles themselves to be defined if you permit multiple
> profiles.
>
> If we dont want to think about this immediately that, then
> we should make $profile=off into a fatal error rather than
> silently ignoring it
I think that makes sense.
I think we can be pretty strict on profiles options. To me it seems
reasonable to say a user can enable **one** profile. Once that profile
is enabled they get all of those extensions.
If possible/simple we can then allow them to manually enable and/or
disable extensions on top of that. I don't see any use in allowing
users to turn profiles "off" though. I'm not even clear what that
means.
Alistair
>
> > - $profile=on => only valid for generic CPUs;
> >
> > - $profile=check -> valid for all CPUs, would only check if the CPU implements the profile.
> >
> >
> > I think this is fine. Drew, care to weight in?
>
>
> With regards,
> Daniel
> --
> |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :|
> |: https://libvirt.org -o- https://fstop138.berrange.com :|
> |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
>
>
On 9/29/23 09:52, Daniel P. Berrangé wrote:
> On Fri, Sep 29, 2023 at 09:49:47AM -0300, Daniel Henrique Barboza wrote:
>>
>>
>> On 9/29/23 08:55, Daniel P. Berrangé wrote:
>>> On Fri, Sep 29, 2023 at 08:29:08AM -0300, Daniel Henrique Barboza wrote:
>>>>
>>>>
>>>> On 9/29/23 07:46, Daniel P. Berrangé wrote:
>>>>> On Tue, Sep 26, 2023 at 04:49:44PM -0300, Daniel Henrique Barboza wrote:
>>>>>> Based-on: 20230926183109.165878-1-dbarboza@ventanamicro.com
>>>>>> ("[PATCH 0/2] riscv: add extension properties for all cpus")
>>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> These patches implements the base profile support for qemu-riscv and the
>>>>>> first profile, RVA22U64.
>>>>>>
>>>>>> As discussed in this thread [1] we're aiming for a flag that enables all
>>>>>> mandatory extensions of a profile. Optional extensions were left behind
>>>>>> and must be enabled by hand if desired. Since this is the first profile
>>>>>> we're adding, we'll need to add the base framework as well.
>>>>>>
>>>>>> The RVA22U64 profile was chosen because qemu-riscv implements all its
>>>>>> extensions, both mandatory and optional. That includes 'zicntr' and
>>>>>> 'zihpm', which we support for awhile but aren't adverting to userspace.
>>>>>>
>>>>>> Other design decisions made:
>>>>>>
>>>>>> - disabling a profile flag does nothing, i.e. we won't mass disable
>>>>>> mandatory extensions of the rva22U64 profile if the user sets
>>>>>> rva22u64=false;
>>>>>
>>>>> Why shouldn't this be allowed ?
>>>>>
>>>>> IIUC, a profile is syntactic sugar for a group of features. If
>>>>> we can disable individual features explicitly, why should we
>>>>> not allow use of the profile as sugar to disable them en-mass ?
>>>>
>>>> In theory there's no harm in allowing mass disabling of extensions but, given
>>>> it's a whole profile, we would end up disabling most/all CPU extensions and
>>>> the guest would do nothing.
>>>
>>> True, that is just user error though. They could disable a profile
>>> and then manually re-enable individual features, and thus get a
>>> working system.
>>>
>>>> There is a thread in the ML:
>>>>
>>>> https://lore.kernel.org/qemu-riscv/CABJz62NyVNu4Z1qmCG7MyJkGG_9yWxjUFHHWjmoQEP6unRrHNA@mail.gmail.com/
>>>>
>>>> Where we discussed the possibility of having a minimal CPU extension set. We didn't
>>>> reach a consensus because the definition of "minimal CPU extension set" vary between
>>>> OSes (Linux requires IMAFD, FreeBSD might require something differ).
>>>>
>>>> Assuming we reach a consensus on what a minimal set is, we could allow disabling mass
>>>> extensions via probile but keeping this minimal set, for example. At very least we
>>>> shouldn't allow users to disable 'I' because that would kill the CPU, so RV64I is
>>>> the minimum set that I would assume for now.
>>>
>>> I'd probably just call that user error too.
>>>
>>>>>
>>>>> TL;DR: feature groups are pretty error prone if more than
>>>>> one is listed by the user, or they're combined with individual
>>>>> features.
>>>>>
>>>>>>
>>>>>> - profile support for vendor CPUs consists into checking if the CPU
>>>>>> happens to have the mandatory extensions required for it. In case it
>>>>>> doesn't we'll error out. This is done to follow the same prerogative
>>>>>> we always had of not allowing extensions being enabled for vendor
>>>>>> CPUs;
>>>>>
>>>>> Why shouldn't this be allowed ?
>>>>
>>>> There's no technical reason to not allow it. The reason it's forbid is to be
>>>> closer to what the real hardware would do. E.g. the real hardware doesn't allow
>>>> users to enable Vector if the hardware doesn't support it. Vendor CPUs also has
>>>> a privileged spec restriction as well, so if a CPU is running in an older spec
>>>> it can't enable extensions that were added later.
>>>
>>> Real hardware is constrained in not being able to invent arbitrary
>>> new features on chip. Virtual machines are not constrained, so
>>> I don't think the inability of hardware todo this, is an especially
>>> strong reason to limit software emulation.
>>>
>>> What I don't like about this, is that (IIUC) the '$profile=on' option
>>> now has different semantics depending on what CPU it is used with.
>>>
>>> ie using it with a vendor CPU, $profile=on becomes an assertion
>>> that the vendor CPU contains all the features needed to satisfy
>>> $profile. It won't enable/disable anything, just check it is present.
>>>
>>> With a non-vendor CPU, using $profile=on becomes a mechanism to force
>>> enable all the features needed to satisfy $profile, there is no
>>> mechanism to just check for presence.
>>>
>>> Having two different semantics for the same syntax is generally considered
>>> bad design practice.
>>>
>>> This points towards supporting a tri-state, not boolean. $profile=check
>>> for validation only, and $profile=on for force enablement.
>>
>> This would leave us with:
>>
>> - $profile=off => disable all extensions. Let users hit themselves in the foot if they
>> don't enable any other extensions. Note that disabling a profile and enabling extensions
>> on top of it is very sensitive to left-to-right ordering, so it would be good to have
>> a way to enforce this ordering somehow (feature groups always first);
>
> It is also order sensitive if 2 profiles have overlap in the
> extensions they represent. So might also require an ordering
> of profiles themselves to be defined if you permit multiple
> profiles.
>
> If we dont want to think about this immediately that, then
> we should make $profile=off into a fatal error rather than
> silently ignoring it
I don't mind handling it right now, I just don't know how hehe
I'll re-read the thread you sent earlier and see if there's something I missed. I got the
impression that we need your qom patch first.
Thanks,
Daniel
>
>> - $profile=on => only valid for generic CPUs;
>>
>> - $profile=check -> valid for all CPUs, would only check if the CPU implements the profile.
>>
>>
>> I think this is fine. Drew, care to weight in?
>
>
> With regards,
> Daniel
On Fri, Sep 29, 2023 at 10:26:08AM -0300, Daniel Henrique Barboza wrote:
>
>
> On 9/29/23 09:52, Daniel P. Berrangé wrote:
> > On Fri, Sep 29, 2023 at 09:49:47AM -0300, Daniel Henrique Barboza wrote:
> > >
> > >
> > > On 9/29/23 08:55, Daniel P. Berrangé wrote:
> > > > On Fri, Sep 29, 2023 at 08:29:08AM -0300, Daniel Henrique Barboza wrote:
> > > > >
> > > > >
> > > > > On 9/29/23 07:46, Daniel P. Berrangé wrote:
> > > > > > On Tue, Sep 26, 2023 at 04:49:44PM -0300, Daniel Henrique Barboza wrote:
> > > > > > > Based-on: 20230926183109.165878-1-dbarboza@ventanamicro.com
> > > > > > > ("[PATCH 0/2] riscv: add extension properties for all cpus")
> > > > > > >
> > > > > > > Hi,
> > > > > > >
> > > > > > > These patches implements the base profile support for qemu-riscv and the
> > > > > > > first profile, RVA22U64.
> > > > > > >
> > > > > > > As discussed in this thread [1] we're aiming for a flag that enables all
> > > > > > > mandatory extensions of a profile. Optional extensions were left behind
> > > > > > > and must be enabled by hand if desired. Since this is the first profile
> > > > > > > we're adding, we'll need to add the base framework as well.
> > > > > > >
> > > > > > > The RVA22U64 profile was chosen because qemu-riscv implements all its
> > > > > > > extensions, both mandatory and optional. That includes 'zicntr' and
> > > > > > > 'zihpm', which we support for awhile but aren't adverting to userspace.
> > > > > > >
> > > > > > > Other design decisions made:
> > > > > > >
> > > > > > > - disabling a profile flag does nothing, i.e. we won't mass disable
> > > > > > > mandatory extensions of the rva22U64 profile if the user sets
> > > > > > > rva22u64=false;
> > > > > >
> > > > > > Why shouldn't this be allowed ?
> > > > > >
> > > > > > IIUC, a profile is syntactic sugar for a group of features. If
> > > > > > we can disable individual features explicitly, why should we
> > > > > > not allow use of the profile as sugar to disable them en-mass ?
> > > > >
> > > > > In theory there's no harm in allowing mass disabling of extensions but, given
> > > > > it's a whole profile, we would end up disabling most/all CPU extensions and
> > > > > the guest would do nothing.
> > > >
> > > > True, that is just user error though. They could disable a profile
> > > > and then manually re-enable individual features, and thus get a
> > > > working system.
> > > >
> > > > > There is a thread in the ML:
> > > > >
> > > > > https://lore.kernel.org/qemu-riscv/CABJz62NyVNu4Z1qmCG7MyJkGG_9yWxjUFHHWjmoQEP6unRrHNA@mail.gmail.com/
> > > > >
> > > > > Where we discussed the possibility of having a minimal CPU extension set. We didn't
> > > > > reach a consensus because the definition of "minimal CPU extension set" vary between
> > > > > OSes (Linux requires IMAFD, FreeBSD might require something differ).
> > > > >
> > > > > Assuming we reach a consensus on what a minimal set is, we could allow disabling mass
> > > > > extensions via probile but keeping this minimal set, for example. At very least we
> > > > > shouldn't allow users to disable 'I' because that would kill the CPU, so RV64I is
> > > > > the minimum set that I would assume for now.
> > > >
> > > > I'd probably just call that user error too.
> > > >
> > > > > >
> > > > > > TL;DR: feature groups are pretty error prone if more than
> > > > > > one is listed by the user, or they're combined with individual
> > > > > > features.
> > > > > >
> > > > > > >
> > > > > > > - profile support for vendor CPUs consists into checking if the CPU
> > > > > > > happens to have the mandatory extensions required for it. In case it
> > > > > > > doesn't we'll error out. This is done to follow the same prerogative
> > > > > > > we always had of not allowing extensions being enabled for vendor
> > > > > > > CPUs;
> > > > > >
> > > > > > Why shouldn't this be allowed ?
> > > > >
> > > > > There's no technical reason to not allow it. The reason it's forbid is to be
> > > > > closer to what the real hardware would do. E.g. the real hardware doesn't allow
> > > > > users to enable Vector if the hardware doesn't support it. Vendor CPUs also has
> > > > > a privileged spec restriction as well, so if a CPU is running in an older spec
> > > > > it can't enable extensions that were added later.
> > > >
> > > > Real hardware is constrained in not being able to invent arbitrary
> > > > new features on chip. Virtual machines are not constrained, so
> > > > I don't think the inability of hardware todo this, is an especially
> > > > strong reason to limit software emulation.
> > > >
> > > > What I don't like about this, is that (IIUC) the '$profile=on' option
> > > > now has different semantics depending on what CPU it is used with.
> > > >
> > > > ie using it with a vendor CPU, $profile=on becomes an assertion
> > > > that the vendor CPU contains all the features needed to satisfy
> > > > $profile. It won't enable/disable anything, just check it is present.
> > > >
> > > > With a non-vendor CPU, using $profile=on becomes a mechanism to force
> > > > enable all the features needed to satisfy $profile, there is no
> > > > mechanism to just check for presence.
> > > >
> > > > Having two different semantics for the same syntax is generally considered
> > > > bad design practice.
> > > >
> > > > This points towards supporting a tri-state, not boolean. $profile=check
> > > > for validation only, and $profile=on for force enablement.
> > >
> > > This would leave us with:
> > >
> > > - $profile=off => disable all extensions. Let users hit themselves in the foot if they
> > > don't enable any other extensions. Note that disabling a profile and enabling extensions
> > > on top of it is very sensitive to left-to-right ordering, so it would be good to have
> > > a way to enforce this ordering somehow (feature groups always first);
> >
> > It is also order sensitive if 2 profiles have overlap in the
> > extensions they represent. So might also require an ordering
> > of profiles themselves to be defined if you permit multiple
> > profiles.
> >
> > If we dont want to think about this immediately that, then
> > we should make $profile=off into a fatal error rather than
> > silently ignoring it
>
> I don't mind handling it right now, I just don't know how hehe
>
> I'll re-read the thread you sent earlier and see if there's something I missed. I got the
> impression that we need your qom patch first.
My patch is dropped as it was considered too gross. Kevin provided
a new impl for ARRAY properties which eliminates the ordering problem.
IOW, QemuOpts iteration order will remain undefined in general.
With regards,
Daniel
--
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|: https://libvirt.org -o- https://fstop138.berrange.com :|
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