Define cpu_emergency_virt_cb even if the kernel is being built without KVM
support so that KVM can reference the typedef in asm/kvm_host.h without
needing yet more #ifdefs.
No functional change intended.
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/include/asm/reboot.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/x86/include/asm/reboot.h b/arch/x86/include/asm/reboot.h
index 6536873f8fc0..d0ef2a678d66 100644
--- a/arch/x86/include/asm/reboot.h
+++ b/arch/x86/include/asm/reboot.h
@@ -25,8 +25,8 @@ void __noreturn machine_real_restart(unsigned int type);
#define MRR_BIOS 0
#define MRR_APM 1
-#if IS_ENABLED(CONFIG_KVM_INTEL) || IS_ENABLED(CONFIG_KVM_AMD)
typedef void (cpu_emergency_virt_cb)(void);
+#if IS_ENABLED(CONFIG_KVM_INTEL) || IS_ENABLED(CONFIG_KVM_AMD)
void cpu_emergency_register_virt_callback(cpu_emergency_virt_cb *callback);
void cpu_emergency_unregister_virt_callback(cpu_emergency_virt_cb *callback);
void cpu_emergency_disable_virtualization(void);
--
2.44.0.769.g3c40516874-googOn Thu, 2024-04-25 at 16:39 -0700, Sean Christopherson wrote:
> Define cpu_emergency_virt_cb even if the kernel is being built without KVM
> support so that KVM can reference the typedef in asm/kvm_host.h without
> needing yet more #ifdefs.
>
> No functional change intended.
>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> arch/x86/include/asm/reboot.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/reboot.h b/arch/x86/include/asm/reboot.h
> index 6536873f8fc0..d0ef2a678d66 100644
> --- a/arch/x86/include/asm/reboot.h
> +++ b/arch/x86/include/asm/reboot.h
> @@ -25,8 +25,8 @@ void __noreturn machine_real_restart(unsigned int type);
> #define MRR_BIOS 0
> #define MRR_APM 1
>
> -#if IS_ENABLED(CONFIG_KVM_INTEL) || IS_ENABLED(CONFIG_KVM_AMD)
> typedef void (cpu_emergency_virt_cb)(void);
> +#if IS_ENABLED(CONFIG_KVM_INTEL) || IS_ENABLED(CONFIG_KVM_AMD)
> void cpu_emergency_register_virt_callback(cpu_emergency_virt_cb *callback);
> void cpu_emergency_unregister_virt_callback(cpu_emergency_virt_cb *callback);
> void cpu_emergency_disable_virtualization(void);
It looks a little it weird. If other file wants to include
<asm/kvm_host.h> (directly or via <linux/kvm_host.h>) unconditionally then
in general I think <asm/kvm_host.h> or <linux/kvm_host.h> should
have something like:
#ifdef CONFIG_KVM
void func(void);
...
#else
static inline void func(void) {}
#endif
But it seems neither <asm/kvm_host.h> nor <linux/kvm_host.h> has this
pattern.
I tried to build with !CONFIG_KVM with patch 2 in this series, and I got
below error:
In file included from ./include/linux/kvm_host.h:45,
from arch/x86/events/intel/core.c:17:
./arch/x86/include/asm/kvm_host.h:1617:9: error: unknown type name
‘cpu_emergency_virt_cb’
1617 | cpu_emergency_virt_cb *emergency_disable;
| ^~~~~~~~~~~~~~~~~~~~~
Looking at the code, it seems it is because intel_guest_get_msrs() needs
'struct kvm_pmu' (e.g., it accesses the members of 'struct kvm_pmu'). But
it doesn't look the relevant code should be compiled when !CONFIG_KVM.
So looks a better way is to explicitly use #ifdef CONFIG_KVM around the
relevant code in the arch/x86/events/intel/core.c?
And it seems vfio does it in vfio_main.c:
#if IS_ENABLED(CONFIG_KVM)
#include <linux/kvm_host.h>
#endif
#if IS_ENABLED(CONFIG_KVM)
void vfio_device_get_kvm_safe(struct vfio_device *device,
struct kvm *kvm)
{
...
}
...
#endif
The only remaining weird thing is 'struct kvm *kvm' is still used
unconditionally in vfio_main.c, but I think the reason it builds fine with
!CONFIG_KVM is because <linux/vfio.h> declares it explicitly:
struct kvm;
struct iommufd_ctx;
...
So it seems to me that this patch around 'cpu_emergency_virt_cb' is more
like a workaround of existing non-perfect <linux/kvm_host.h> and/or
<asm/kvm_host.h>?
On Mon, May 13, 2024, Kai Huang wrote:
> On Thu, 2024-04-25 at 16:39 -0700, Sean Christopherson wrote:
> > Define cpu_emergency_virt_cb even if the kernel is being built without KVM
> > support so that KVM can reference the typedef in asm/kvm_host.h without
> > needing yet more #ifdefs.
> >
> > No functional change intended.
> >
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> > arch/x86/include/asm/reboot.h | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/include/asm/reboot.h b/arch/x86/include/asm/reboot.h
> > index 6536873f8fc0..d0ef2a678d66 100644
> > --- a/arch/x86/include/asm/reboot.h
> > +++ b/arch/x86/include/asm/reboot.h
> > @@ -25,8 +25,8 @@ void __noreturn machine_real_restart(unsigned int type);
> > #define MRR_BIOS 0
> > #define MRR_APM 1
> >
> > -#if IS_ENABLED(CONFIG_KVM_INTEL) || IS_ENABLED(CONFIG_KVM_AMD)
> > typedef void (cpu_emergency_virt_cb)(void);
> > +#if IS_ENABLED(CONFIG_KVM_INTEL) || IS_ENABLED(CONFIG_KVM_AMD)
> > void cpu_emergency_register_virt_callback(cpu_emergency_virt_cb *callback);
> > void cpu_emergency_unregister_virt_callback(cpu_emergency_virt_cb *callback);
> > void cpu_emergency_disable_virtualization(void);
>
> It looks a little it weird. If other file wants to include
> <asm/kvm_host.h> (directly or via <linux/kvm_host.h>) unconditionally then
> in general I think <asm/kvm_host.h> or <linux/kvm_host.h> should
> have something like:
>
> #ifdef CONFIG_KVM
>
> void func(void);
> ...
>
> #else
>
> static inline void func(void) {}
>
> #endif
>
> But it seems neither <asm/kvm_host.h> nor <linux/kvm_host.h> has this
> pattern.
>
> I tried to build with !CONFIG_KVM with patch 2 in this series, and I got
> below error:
Well, yeah.
> In file included from ./include/linux/kvm_host.h:45,
> from arch/x86/events/intel/core.c:17:
> ./arch/x86/include/asm/kvm_host.h:1617:9: error: unknown type name
> ‘cpu_emergency_virt_cb’
> 1617 | cpu_emergency_virt_cb *emergency_disable;
> | ^~~~~~~~~~~~~~~~~~~~~
>
>
> Looking at the code, it seems it is because intel_guest_get_msrs() needs
> 'struct kvm_pmu' (e.g., it accesses the members of 'struct kvm_pmu'). But
> it doesn't look the relevant code should be compiled when !CONFIG_KVM.
>
> So looks a better way is to explicitly use #ifdef CONFIG_KVM around the
> relevant code in the arch/x86/events/intel/core.c?
Eh, there's no right or wrong way to handle code that is conditionally compiled.
There are always tradeoffs and pros/cons, e.g. the number of #ifdefs, the amount
of effective code validation for all configs, readability, etc.
E.g. if there is only one user of a function that conditionally exists, then
having the caller handle the situation might be cleaner. But if there are
multiple callers, then providing a stub is usually preferable.
IMO, the real problem is that perf pokes into KVM _at all_. Same for VFIO.
The perf usage is especially egregious, as there is zero reason perf should need
KVM internals[1]. VFIO requires a bit more effort, but I'm fairly confident that
Jason's file-based approach[2] will yield clean, robust code that minimizes the
number of #ifdefs required.
I'm planning/hoping to get back to that series in the next few weeks. As for
this small series, I prefer to unconditionally define the typedef, as it requires
no additional #ifdefs, and there are no meaningful downsides to letting the
typedef exist for all kernel builds.
[1] https://lore.kernel.org/all/20230916003118.2540661-21-seanjc@google.com
[2] https://lore.kernel.org/all/ZXkVSKULLivrMkBl@google.com
> And it seems vfio does it in vfio_main.c:
>
> #if IS_ENABLED(CONFIG_KVM)
> #include <linux/kvm_host.h>
> #endif
>
> #if IS_ENABLED(CONFIG_KVM)
> void vfio_device_get_kvm_safe(struct vfio_device *device,
> struct kvm *kvm)
> {
> ...
> }
> ...
> #endif
>
>
> The only remaining weird thing is 'struct kvm *kvm' is still used
> unconditionally in vfio_main.c, but I think the reason it builds fine with
> !CONFIG_KVM is because <linux/vfio.h> declares it explicitly:
>
> struct kvm;
> struct iommufd_ctx;
> ...
>
> So it seems to me that this patch around 'cpu_emergency_virt_cb' is more
> like a workaround of existing non-perfect <linux/kvm_host.h> and/or
> <asm/kvm_host.h>?
On 14/05/2024 4:01 am, Sean Christopherson wrote:
> On Mon, May 13, 2024, Kai Huang wrote:
>> On Thu, 2024-04-25 at 16:39 -0700, Sean Christopherson wrote:
>>> Define cpu_emergency_virt_cb even if the kernel is being built without KVM
>>> support so that KVM can reference the typedef in asm/kvm_host.h without
>>> needing yet more #ifdefs.
>>>
>>> No functional change intended.
>>>
>>> Signed-off-by: Sean Christopherson <seanjc@google.com>
>>> ---
>>> arch/x86/include/asm/reboot.h | 2 +-
>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/include/asm/reboot.h b/arch/x86/include/asm/reboot.h
>>> index 6536873f8fc0..d0ef2a678d66 100644
>>> --- a/arch/x86/include/asm/reboot.h
>>> +++ b/arch/x86/include/asm/reboot.h
>>> @@ -25,8 +25,8 @@ void __noreturn machine_real_restart(unsigned int type);
>>> #define MRR_BIOS 0
>>> #define MRR_APM 1
>>>
>>> -#if IS_ENABLED(CONFIG_KVM_INTEL) || IS_ENABLED(CONFIG_KVM_AMD)
>>> typedef void (cpu_emergency_virt_cb)(void);
>>> +#if IS_ENABLED(CONFIG_KVM_INTEL) || IS_ENABLED(CONFIG_KVM_AMD)
>>> void cpu_emergency_register_virt_callback(cpu_emergency_virt_cb *callback);
>>> void cpu_emergency_unregister_virt_callback(cpu_emergency_virt_cb *callback);
>>> void cpu_emergency_disable_virtualization(void);
>>
>> It looks a little it weird. If other file wants to include
>> <asm/kvm_host.h> (directly or via <linux/kvm_host.h>) unconditionally then
>> in general I think <asm/kvm_host.h> or <linux/kvm_host.h> should
>> have something like:
>>
>> #ifdef CONFIG_KVM
>>
>> void func(void);
>> ...
>>
>> #else
>>
>> static inline void func(void) {}
>>
>> #endif
>>
>> But it seems neither <asm/kvm_host.h> nor <linux/kvm_host.h> has this
>> pattern.
>>
>> I tried to build with !CONFIG_KVM with patch 2 in this series, and I got
>> below error:
>
> Well, yeah.
>
>> In file included from ./include/linux/kvm_host.h:45,
>> from arch/x86/events/intel/core.c:17:
>> ./arch/x86/include/asm/kvm_host.h:1617:9: error: unknown type name
>> ‘cpu_emergency_virt_cb’
>> 1617 | cpu_emergency_virt_cb *emergency_disable;
>> | ^~~~~~~~~~~~~~~~~~~~~
>>
>>
>> Looking at the code, it seems it is because intel_guest_get_msrs() needs
>> 'struct kvm_pmu' (e.g., it accesses the members of 'struct kvm_pmu'). But
>> it doesn't look the relevant code should be compiled when !CONFIG_KVM.
>>
>> So looks a better way is to explicitly use #ifdef CONFIG_KVM around the
>> relevant code in the arch/x86/events/intel/core.c?
>
> Eh, there's no right or wrong way to handle code that is conditionally compiled.
> There are always tradeoffs and pros/cons, e.g. the number of #ifdefs, the amount
> of effective code validation for all configs, readability, etc.
>
> E.g. if there is only one user of a function that conditionally exists, then
> having the caller handle the situation might be cleaner. But if there are
> multiple callers, then providing a stub is usually preferable.
Yeah.
>
> IMO, the real problem is that perf pokes into KVM _at all_. Same for VFIO.
> The perf usage is especially egregious, as there is zero reason perf should need
> KVM internals[1]. VFIO requires a bit more effort, but I'm fairly confident that
> Jason's file-based approach[2] will yield clean, robust code that minimizes the
> number of #ifdefs required.
>
> I'm planning/hoping to get back to that series in the next few weeks. As for
> this small series, I prefer to unconditionally define the typedef, as it requires
> no additional #ifdefs, and there are no meaningful downsides to letting the
> typedef exist for all kernel builds.
Seems the final target is to remove those <linux/kvm_host.h> users, or I
think a safe-once-for-all solution is to provide the stubs in
<linux/kvm_host.h> with:
#ifdef CONFIG_KVM
...
#else
#endif
In either way, my concerns is it seems modifying the <asm/reboot.h> is a
temporary workaround. And when we reach the final solution I suppose we
will need to revert it back to the current way?
If so, how about manually add a temporary typedef in <asm/kvm_host.h>
for now?
#ifndef CONFIG_KVM
typedef void (cpu_emergency_virt_cb)(void);
#endif
Yes it's ugly, but it's KVM self-contained, and can be removed when ready.
Anyway, just my 2 cents.
>
> [1] https://lore.kernel.org/all/20230916003118.2540661-21-seanjc@google.com
> [2] https://lore.kernel.org/all/ZXkVSKULLivrMkBl@google.com
>
>> And it seems vfio does it in vfio_main.c:
>>
>> #if IS_ENABLED(CONFIG_KVM)
>> #include <linux/kvm_host.h>
>> #endif
>>
>> #if IS_ENABLED(CONFIG_KVM)
>> void vfio_device_get_kvm_safe(struct vfio_device *device,
>> struct kvm *kvm)
>> {
>> ...
>> }
>> ...
>> #endif
>>
>>
>> The only remaining weird thing is 'struct kvm *kvm' is still used
>> unconditionally in vfio_main.c, but I think the reason it builds fine with
>> !CONFIG_KVM is because <linux/vfio.h> declares it explicitly:
>>
>> struct kvm;
>> struct iommufd_ctx;
>> ...
>>
>> So it seems to me that this patch around 'cpu_emergency_virt_cb' is more
>> like a workaround of existing non-perfect <linux/kvm_host.h> and/or
>> <asm/kvm_host.h>?
>
On 14/05/2024 10:44 am, Huang, Kai wrote:
>
>
> On 14/05/2024 4:01 am, Sean Christopherson wrote:
>> On Mon, May 13, 2024, Kai Huang wrote:
>>> On Thu, 2024-04-25 at 16:39 -0700, Sean Christopherson wrote:
>>>> Define cpu_emergency_virt_cb even if the kernel is being built
>>>> without KVM
>>>> support so that KVM can reference the typedef in asm/kvm_host.h without
>>>> needing yet more #ifdefs.
>>>>
>>>> No functional change intended.
>>>>
>>>> Signed-off-by: Sean Christopherson <seanjc@google.com>
>>>> ---
>>>> arch/x86/include/asm/reboot.h | 2 +-
>>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/arch/x86/include/asm/reboot.h
>>>> b/arch/x86/include/asm/reboot.h
>>>> index 6536873f8fc0..d0ef2a678d66 100644
>>>> --- a/arch/x86/include/asm/reboot.h
>>>> +++ b/arch/x86/include/asm/reboot.h
>>>> @@ -25,8 +25,8 @@ void __noreturn machine_real_restart(unsigned int
>>>> type);
>>>> #define MRR_BIOS 0
>>>> #define MRR_APM 1
>>>> -#if IS_ENABLED(CONFIG_KVM_INTEL) || IS_ENABLED(CONFIG_KVM_AMD)
>>>> typedef void (cpu_emergency_virt_cb)(void);
>>>> +#if IS_ENABLED(CONFIG_KVM_INTEL) || IS_ENABLED(CONFIG_KVM_AMD)
>>>> void cpu_emergency_register_virt_callback(cpu_emergency_virt_cb
>>>> *callback);
>>>> void cpu_emergency_unregister_virt_callback(cpu_emergency_virt_cb
>>>> *callback);
>>>> void cpu_emergency_disable_virtualization(void);
>>>
>>> It looks a little it weird. If other file wants to include
>>> <asm/kvm_host.h> (directly or via <linux/kvm_host.h>) unconditionally
>>> then
>>> in general I think <asm/kvm_host.h> or <linux/kvm_host.h> should
>>> have something like:
>>>
>>> #ifdef CONFIG_KVM
>>>
>>> void func(void);
>>> ...
>>>
>>> #else
>>>
>>> static inline void func(void) {}
>>>
>>> #endif
>>>
>>> But it seems neither <asm/kvm_host.h> nor <linux/kvm_host.h> has this
>>> pattern.
>>>
>>> I tried to build with !CONFIG_KVM with patch 2 in this series, and I got
>>> below error:
>>
>> Well, yeah.
>>
>>> In file included from ./include/linux/kvm_host.h:45,
>>> from arch/x86/events/intel/core.c:17:
>>> ./arch/x86/include/asm/kvm_host.h:1617:9: error: unknown type name
>>> ‘cpu_emergency_virt_cb’
>>> 1617 | cpu_emergency_virt_cb *emergency_disable;
>>> | ^~~~~~~~~~~~~~~~~~~~~
>>>
>>>
>>> Looking at the code, it seems it is because intel_guest_get_msrs() needs
>>> 'struct kvm_pmu' (e.g., it accesses the members of 'struct
>>> kvm_pmu'). But
>>> it doesn't look the relevant code should be compiled when !CONFIG_KVM.
>>>
>>> So looks a better way is to explicitly use #ifdef CONFIG_KVM around the
>>> relevant code in the arch/x86/events/intel/core.c?
>>
>> Eh, there's no right or wrong way to handle code that is conditionally
>> compiled.
>> There are always tradeoffs and pros/cons, e.g. the number of #ifdefs,
>> the amount
>> of effective code validation for all configs, readability, etc.
>>
>> E.g. if there is only one user of a function that conditionally
>> exists, then
>> having the caller handle the situation might be cleaner. But if there
>> are
>> multiple callers, then providing a stub is usually preferable.
>
> Yeah.
>
>>
>> IMO, the real problem is that perf pokes into KVM _at all_. Same for
>> VFIO.
>> The perf usage is especially egregious, as there is zero reason perf
>> should need
>> KVM internals[1]. VFIO requires a bit more effort, but I'm fairly
>> confident that
>> Jason's file-based approach[2] will yield clean, robust code that
>> minimizes the
>> number of #ifdefs required.
>>
>> I'm planning/hoping to get back to that series in the next few weeks.
>> As for
>> this small series, I prefer to unconditionally define the typedef, as
>> it requires
>> no additional #ifdefs, and there are no meaningful downsides to
>> letting the
>> typedef exist for all kernel builds.
>
> Seems the final target is to remove those <linux/kvm_host.h> users, or I
> think a safe-once-for-all solution is to provide the stubs in
> <linux/kvm_host.h> with:
>
> #ifdef CONFIG_KVM
> ...
> #else
> #endif
>
> In either way, my concerns is it seems modifying the <asm/reboot.h> is a
> temporary workaround. And when we reach the final solution I suppose we
> will need to revert it back to the current way?
>
> If so, how about manually add a temporary typedef in <asm/kvm_host.h>
> for now?
>
> #ifndef CONFIG_KVM
> typedef void (cpu_emergency_virt_cb)(void);
> #endif
>
> Yes it's ugly, but it's KVM self-contained, and can be removed when ready.
>
> Anyway, just my 2 cents.
>
A second thought:
How about we just make all emergency virtualization disable code
unconditional but not guided by CONFIG_KVM_INTEL || CONFIG_KVM_AMD,
i.e., revert commit
261cd5ed934e ("x86/reboot: Expose VMCS crash hooks if and only if
KVM_{INTEL,AMD} is enabled")
It makes sense anyway from the perspective that it allows the
out-of-tree kernel module hypervisor to use this mechanism w/o needing
to have the kernel built with KVM enabled in Kconfig. Otherwise,
strictly speaking, IIUC, the kernel won't be able to support out-of-tree
module hypervisor as there's no other way the module can intercept
emergency reboot.
This approach avoids the weirdness of the unconditional define for only
cpu_emergency_virt_cb.
On Wed, May 15, 2024, Kai Huang wrote:
> How about we just make all emergency virtualization disable code
> unconditional but not guided by CONFIG_KVM_INTEL || CONFIG_KVM_AMD, i.e.,
> revert commit
>
> 261cd5ed934e ("x86/reboot: Expose VMCS crash hooks if and only if
> KVM_{INTEL,AMD} is enabled")
>
> It makes sense anyway from the perspective that it allows the out-of-tree
> kernel module hypervisor to use this mechanism w/o needing to have the
> kernel built with KVM enabled in Kconfig. Otherwise, strictly speaking,
> IIUC, the kernel won't be able to support out-of-tree module hypervisor as
> there's no other way the module can intercept emergency reboot.
Practically speaking, no one is running an out-of-tree hypervisor without either
(a) KVM being enabled in the .config, or (b) non-trivial changes to the kernel.
Exposing/exporting select APIs and symbols if and only if KVM is enabled is a
a well-established pattern, and there are concrete benefits to doing so. E.g.
it allows minimizing the kernel footprint for use cases that don't want/need KVM.
> This approach avoids the weirdness of the unconditional define for only
> cpu_emergency_virt_cb.
I genuinely don't understand why you find it weird to unconditionally define
cpu_emergency_virt_cb. There are myriad examples throughout the kernel where a
typedef, struct, enum, etc. is declared/defined even though support for its sole
end consumer is disabled. E.g. include/linux/mm_types.h declares "struct mem_cgroup"
for pretty much the exact same reason, even though the structure is only fully
defined if CONFIG_MEMCG=y.
The only oddity here is that the API that the #ifdef that guards the usage happens
to be right below the typedef, but it shouldn't take that much brain power to
figure out why a typedef exists outside of an #ifdef.
On 22/05/2024 8:02 am, Sean Christopherson wrote:
> On Wed, May 15, 2024, Kai Huang wrote:
>> How about we just make all emergency virtualization disable code
>> unconditional but not guided by CONFIG_KVM_INTEL || CONFIG_KVM_AMD, i.e.,
>> revert commit
>>
>> 261cd5ed934e ("x86/reboot: Expose VMCS crash hooks if and only if
>> KVM_{INTEL,AMD} is enabled")
>>
>> It makes sense anyway from the perspective that it allows the out-of-tree
>> kernel module hypervisor to use this mechanism w/o needing to have the
>> kernel built with KVM enabled in Kconfig. Otherwise, strictly speaking,
>> IIUC, the kernel won't be able to support out-of-tree module hypervisor as
>> there's no other way the module can intercept emergency reboot.
>
> Practically speaking, no one is running an out-of-tree hypervisor without either
> (a) KVM being enabled in the .config, or (b) non-trivial changes to the kernel.
Just for curiosity: why b) is required to support out-of-tree hypervisor
when KVM is disabled in Kconfig? I am probably missing something.
>
> Exposing/exporting select APIs and symbols if and only if KVM is enabled is a
> a well-established pattern, and there are concrete benefits to doing so. E.g.
> it allows minimizing the kernel footprint for use cases that don't want/need KVM.
>
>> This approach avoids the weirdness of the unconditional define for only
>> cpu_emergency_virt_cb.
>
> I genuinely don't understand why you find it weird to unconditionally define
> cpu_emergency_virt_cb. There are myriad examples throughout the kernel where a
> typedef, struct, enum, etc. is declared/defined even though support for its sole
> end consumer is disabled. E.g. include/linux/mm_types.h declares "struct mem_cgroup"
> for pretty much the exact same reason, even though the structure is only fully
> defined if CONFIG_MEMCG=y.
>
> The only oddity here is that the API that the #ifdef that guards the usage happens
> to be right below the typedef, but it shouldn't take that much brain power to
> figure out why a typedef exists outside of an #ifdef.
OK. No more arguments. :-)
Thanks for this series anyway.
On Wed, May 22, 2024, Kai Huang wrote:
> On 22/05/2024 8:02 am, Sean Christopherson wrote:
> > On Wed, May 15, 2024, Kai Huang wrote:
> > > How about we just make all emergency virtualization disable code
> > > unconditional but not guided by CONFIG_KVM_INTEL || CONFIG_KVM_AMD, i.e.,
> > > revert commit
> > >
> > > 261cd5ed934e ("x86/reboot: Expose VMCS crash hooks if and only if
> > > KVM_{INTEL,AMD} is enabled")
> > >
> > > It makes sense anyway from the perspective that it allows the out-of-tree
> > > kernel module hypervisor to use this mechanism w/o needing to have the
> > > kernel built with KVM enabled in Kconfig. Otherwise, strictly speaking,
> > > IIUC, the kernel won't be able to support out-of-tree module hypervisor as
> > > there's no other way the module can intercept emergency reboot.
> >
> > Practically speaking, no one is running an out-of-tree hypervisor without either
> > (a) KVM being enabled in the .config, or (b) non-trivial changes to the kernel.
>
> Just for curiosity: why b) is required to support out-of-tree hypervisor
> when KVM is disabled in Kconfig? I am probably missing something.
A variety of hooks that are likely needed for any x86 hypervisor (especially on
Intel) are guarded by CONFIG_KVM. E.g. the posted interrupt vectors (though it's
definitely possible to use a different model than KVM), the entry point for
trampolining NMIs (though again, a hypervisor could just do "INT 2", at least
until FRED comes along), and probably a few other things.
I'm sure it's possible to workaround any issues, but I would be quite surprised
if out-of-tree code went through the effort of functioning with a kernel built
to play nice with KVM. Who knows, maybe I'm entirely wrong :-)
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