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[PATCH v2 2/9] kvm: Detect guest-memfd flags supported

Peter Xu posted 9 patches 2 months, 3 weeks ago
Maintainers: Paolo Bonzini <pbonzini@redhat.com>, David Hildenbrand <david@kernel.org>, Igor Mammedov <imammedo@redhat.com>, Peter Xu <peterx@redhat.com>, "Philippe Mathieu-Daudé" <philmd@linaro.org>, Eric Blake <eblake@redhat.com>, Markus Armbruster <armbru@redhat.com>, "Daniel P. Berrangé" <berrange@redhat.com>, Eduardo Habkost <eduardo@habkost.net>, Fabiano Rosas <farosas@suse.de>, Laurent Vivier <lvivier@redhat.com>
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[PATCH v2 2/9] kvm: Detect guest-memfd flags supported
Posted by Peter Xu 2 months, 3 weeks ago 
Detect supported guest-memfd flags by the current kernel, and reject
creations of guest-memfd using invalid flags.  When the cap isn't
available, then no flag is supported.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 accel/kvm/kvm-all.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index 96c194ce54..f477014126 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -108,6 +108,7 @@ static int kvm_sstep_flags;
 static bool kvm_immediate_exit;
 static uint64_t kvm_supported_memory_attributes;
 static bool kvm_guest_memfd_supported;
+static uint64_t kvm_guest_memfd_flags_supported;
 static hwaddr kvm_max_slot_size = ~0;
 
 static const KVMCapabilityInfo kvm_required_capabilites[] = {
@@ -2787,6 +2788,10 @@ static int kvm_init(AccelState *as, MachineState *ms)
     kvm_guest_memfd_supported =
         kvm_vm_check_extension(s, KVM_CAP_GUEST_MEMFD) &&
         kvm_vm_check_extension(s, KVM_CAP_USER_MEMORY2);
+
+    ret = kvm_vm_check_extension(s, KVM_CAP_GUEST_MEMFD_FLAGS);
+    kvm_guest_memfd_flags_supported = ret > 0 ? ret : 0;
+
     kvm_pre_fault_memory_supported = kvm_vm_check_extension(s, KVM_CAP_PRE_FAULT_MEMORY);
 
     if (s->kernel_irqchip_split == ON_OFF_AUTO_AUTO) {
@@ -4492,6 +4497,12 @@ int kvm_create_guest_memfd(uint64_t size, uint64_t flags, Error **errp)
         return -1;
     }
 
+    if (flags & ~kvm_guest_memfd_flags_supported) {
+        error_setg(errp, "KVM does not support guest-memfd flag: 0x%"PRIx64,
+                   flags & ~kvm_guest_memfd_flags_supported);
+        return -1;
+    }
+
     fd = kvm_vm_ioctl(kvm_state, KVM_CREATE_GUEST_MEMFD, &guest_memfd);
     if (fd < 0) {
         error_setg_errno(errp, errno, "Error creating KVM guest_memfd");
-- 
2.50.1
Re: [PATCH v2 2/9] kvm: Detect guest-memfd flags supported
Posted by Xiaoyao Li 1 month, 4 weeks ago 
On 11/20/2025 1:29 AM, Peter Xu wrote:
> Detect supported guest-memfd flags by the current kernel, and reject
> creations of guest-memfd using invalid flags.  When the cap isn't
> available, then no flag is supported.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>   accel/kvm/kvm-all.c | 11 +++++++++++
>   1 file changed, 11 insertions(+)
> 
> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> index 96c194ce54..f477014126 100644
> --- a/accel/kvm/kvm-all.c
> +++ b/accel/kvm/kvm-all.c
> @@ -108,6 +108,7 @@ static int kvm_sstep_flags;
>   static bool kvm_immediate_exit;
>   static uint64_t kvm_supported_memory_attributes;
>   static bool kvm_guest_memfd_supported;
> +static uint64_t kvm_guest_memfd_flags_supported;
>   static hwaddr kvm_max_slot_size = ~0;
>   
>   static const KVMCapabilityInfo kvm_required_capabilites[] = {
> @@ -2787,6 +2788,10 @@ static int kvm_init(AccelState *as, MachineState *ms)
>       kvm_guest_memfd_supported =
>           kvm_vm_check_extension(s, KVM_CAP_GUEST_MEMFD) &&
>           kvm_vm_check_extension(s, KVM_CAP_USER_MEMORY2);
> +
> +    ret = kvm_vm_check_extension(s, KVM_CAP_GUEST_MEMFD_FLAGS);
> +    kvm_guest_memfd_flags_supported = ret > 0 ? ret : 0;
> +
>       kvm_pre_fault_memory_supported = kvm_vm_check_extension(s, KVM_CAP_PRE_FAULT_MEMORY);
>   
>       if (s->kernel_irqchip_split == ON_OFF_AUTO_AUTO) {
> @@ -4492,6 +4497,12 @@ int kvm_create_guest_memfd(uint64_t size, uint64_t flags, Error **errp)
>           return -1;
>       }
>   
> +    if (flags & ~kvm_guest_memfd_flags_supported) {
> +        error_setg(errp, "KVM does not support guest-memfd flag: 0x%"PRIx64,

I'm thinking if need to add "for current VM" like:

	KVM does not support guest-memfd flag: xxx for current VM

because kvm_guest_memfd_flags_supported is got from VM-scope CAP and 
varies for different VM types.

> +                   flags & ~kvm_guest_memfd_flags_supported);
> +        return -1;
> +    }
> +
>       fd = kvm_vm_ioctl(kvm_state, KVM_CREATE_GUEST_MEMFD, &guest_memfd);
>       if (fd < 0) {
>           error_setg_errno(errp, errno, "Error creating KVM guest_memfd");
Re: [PATCH v2 2/9] kvm: Detect guest-memfd flags supported
Posted by Peter Xu 1 month, 4 weeks ago 
On Fri, Dec 12, 2025 at 11:10:23AM +0800, Xiaoyao Li wrote:
> On 11/20/2025 1:29 AM, Peter Xu wrote:
> > Detect supported guest-memfd flags by the current kernel, and reject
> > creations of guest-memfd using invalid flags.  When the cap isn't
> > available, then no flag is supported.
> > 
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> >   accel/kvm/kvm-all.c | 11 +++++++++++
> >   1 file changed, 11 insertions(+)
> > 
> > diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> > index 96c194ce54..f477014126 100644
> > --- a/accel/kvm/kvm-all.c
> > +++ b/accel/kvm/kvm-all.c
> > @@ -108,6 +108,7 @@ static int kvm_sstep_flags;
> >   static bool kvm_immediate_exit;
> >   static uint64_t kvm_supported_memory_attributes;
> >   static bool kvm_guest_memfd_supported;
> > +static uint64_t kvm_guest_memfd_flags_supported;
> >   static hwaddr kvm_max_slot_size = ~0;
> >   static const KVMCapabilityInfo kvm_required_capabilites[] = {
> > @@ -2787,6 +2788,10 @@ static int kvm_init(AccelState *as, MachineState *ms)
> >       kvm_guest_memfd_supported =
> >           kvm_vm_check_extension(s, KVM_CAP_GUEST_MEMFD) &&
> >           kvm_vm_check_extension(s, KVM_CAP_USER_MEMORY2);
> > +
> > +    ret = kvm_vm_check_extension(s, KVM_CAP_GUEST_MEMFD_FLAGS);
> > +    kvm_guest_memfd_flags_supported = ret > 0 ? ret : 0;
> > +
> >       kvm_pre_fault_memory_supported = kvm_vm_check_extension(s, KVM_CAP_PRE_FAULT_MEMORY);
> >       if (s->kernel_irqchip_split == ON_OFF_AUTO_AUTO) {
> > @@ -4492,6 +4497,12 @@ int kvm_create_guest_memfd(uint64_t size, uint64_t flags, Error **errp)
> >           return -1;
> >       }
> > +    if (flags & ~kvm_guest_memfd_flags_supported) {
> > +        error_setg(errp, "KVM does not support guest-memfd flag: 0x%"PRIx64,
> 
> I'm thinking if need to add "for current VM" like:
> 
> 	KVM does not support guest-memfd flag: xxx for current VM
> 
> because kvm_guest_memfd_flags_supported is got from VM-scope CAP and varies
> for different VM types.

Sure I can amend it.  Though I plan to change the sentence slightly:

     if (flags & ~kvm_guest_memfd_flags_supported) {
-        error_setg(errp, "KVM does not support guest-memfd flag: 0x%"PRIx64,
+        error_setg(errp, "Current KVM instance does not support "
+                   "guest-memfd flag: 0x%"PRIx64,
                    flags & ~kvm_guest_memfd_flags_supported);
         return -1;
     }

Thanks,

> 
> > +                   flags & ~kvm_guest_memfd_flags_supported);
> > +        return -1;
> > +    }
> > +
> >       fd = kvm_vm_ioctl(kvm_state, KVM_CREATE_GUEST_MEMFD, &guest_memfd);
> >       if (fd < 0) {
> >           error_setg_errno(errp, errno, "Error creating KVM guest_memfd");
> 

-- 
Peter Xu
Re: [PATCH v2 2/9] kvm: Detect guest-memfd flags supported
Posted by Xiaoyao Li 1 month, 3 weeks ago 
On 12/13/2025 1:23 AM, Peter Xu wrote:
> On Fri, Dec 12, 2025 at 11:10:23AM +0800, Xiaoyao Li wrote:
>> On 11/20/2025 1:29 AM, Peter Xu wrote:
>>> Detect supported guest-memfd flags by the current kernel, and reject
>>> creations of guest-memfd using invalid flags.  When the cap isn't
>>> available, then no flag is supported.
>>>
>>> Signed-off-by: Peter Xu <peterx@redhat.com>
>>> ---
>>>    accel/kvm/kvm-all.c | 11 +++++++++++
>>>    1 file changed, 11 insertions(+)
>>>
>>> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
>>> index 96c194ce54..f477014126 100644
>>> --- a/accel/kvm/kvm-all.c
>>> +++ b/accel/kvm/kvm-all.c
>>> @@ -108,6 +108,7 @@ static int kvm_sstep_flags;
>>>    static bool kvm_immediate_exit;
>>>    static uint64_t kvm_supported_memory_attributes;
>>>    static bool kvm_guest_memfd_supported;
>>> +static uint64_t kvm_guest_memfd_flags_supported;
>>>    static hwaddr kvm_max_slot_size = ~0;
>>>    static const KVMCapabilityInfo kvm_required_capabilites[] = {
>>> @@ -2787,6 +2788,10 @@ static int kvm_init(AccelState *as, MachineState *ms)
>>>        kvm_guest_memfd_supported =
>>>            kvm_vm_check_extension(s, KVM_CAP_GUEST_MEMFD) &&
>>>            kvm_vm_check_extension(s, KVM_CAP_USER_MEMORY2);
>>> +
>>> +    ret = kvm_vm_check_extension(s, KVM_CAP_GUEST_MEMFD_FLAGS);
>>> +    kvm_guest_memfd_flags_supported = ret > 0 ? ret : 0;
>>> +
>>>        kvm_pre_fault_memory_supported = kvm_vm_check_extension(s, KVM_CAP_PRE_FAULT_MEMORY);
>>>        if (s->kernel_irqchip_split == ON_OFF_AUTO_AUTO) {
>>> @@ -4492,6 +4497,12 @@ int kvm_create_guest_memfd(uint64_t size, uint64_t flags, Error **errp)
>>>            return -1;
>>>        }
>>> +    if (flags & ~kvm_guest_memfd_flags_supported) {
>>> +        error_setg(errp, "KVM does not support guest-memfd flag: 0x%"PRIx64,
>>
>> I'm thinking if need to add "for current VM" like:
>>
>> 	KVM does not support guest-memfd flag: xxx for current VM
>>
>> because kvm_guest_memfd_flags_supported is got from VM-scope CAP and varies
>> for different VM types.
> 
> Sure I can amend it.  Though I plan to change the sentence slightly:
> 
>       if (flags & ~kvm_guest_memfd_flags_supported) {
> -        error_setg(errp, "KVM does not support guest-memfd flag: 0x%"PRIx64,
> +        error_setg(errp, "Current KVM instance does not support "
> +                   "guest-memfd flag: 0x%"PRIx64,
>                      flags & ~kvm_guest_memfd_flags_supported);
>           return -1;
>       }

Works for me.

> Thanks,
> 
>>
>>> +                   flags & ~kvm_guest_memfd_flags_supported);
>>> +        return -1;
>>> +    }
>>> +
>>>        fd = kvm_vm_ioctl(kvm_state, KVM_CREATE_GUEST_MEMFD, &guest_memfd);
>>>        if (fd < 0) {
>>>            error_setg_errno(errp, errno, "Error creating KVM guest_memfd");
>>
>
Re: [PATCH v2 2/9] kvm: Detect guest-memfd flags supported
Posted by Xiaoyao Li 1 month, 4 weeks ago 
On 11/20/2025 1:29 AM, Peter Xu wrote:
> Detect supported guest-memfd flags by the current kernel, and reject
> creations of guest-memfd using invalid flags.  When the cap isn't
> available, then no flag is supported.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>

Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>

> ---
>   accel/kvm/kvm-all.c | 11 +++++++++++
>   1 file changed, 11 insertions(+)
> 
> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> index 96c194ce54..f477014126 100644
> --- a/accel/kvm/kvm-all.c
> +++ b/accel/kvm/kvm-all.c
> @@ -108,6 +108,7 @@ static int kvm_sstep_flags;
>   static bool kvm_immediate_exit;
>   static uint64_t kvm_supported_memory_attributes;
>   static bool kvm_guest_memfd_supported;
> +static uint64_t kvm_guest_memfd_flags_supported;
>   static hwaddr kvm_max_slot_size = ~0;
>   
>   static const KVMCapabilityInfo kvm_required_capabilites[] = {
> @@ -2787,6 +2788,10 @@ static int kvm_init(AccelState *as, MachineState *ms)
>       kvm_guest_memfd_supported =
>           kvm_vm_check_extension(s, KVM_CAP_GUEST_MEMFD) &&
>           kvm_vm_check_extension(s, KVM_CAP_USER_MEMORY2);
> +
> +    ret = kvm_vm_check_extension(s, KVM_CAP_GUEST_MEMFD_FLAGS);
> +    kvm_guest_memfd_flags_supported = ret > 0 ? ret : 0;
> +
>       kvm_pre_fault_memory_supported = kvm_vm_check_extension(s, KVM_CAP_PRE_FAULT_MEMORY);
>   
>       if (s->kernel_irqchip_split == ON_OFF_AUTO_AUTO) {
> @@ -4492,6 +4497,12 @@ int kvm_create_guest_memfd(uint64_t size, uint64_t flags, Error **errp)
>           return -1;
>       }
>   
> +    if (flags & ~kvm_guest_memfd_flags_supported) {
> +        error_setg(errp, "KVM does not support guest-memfd flag: 0x%"PRIx64,
> +                   flags & ~kvm_guest_memfd_flags_supported);
> +        return -1;
> +    }
> +
>       fd = kvm_vm_ioctl(kvm_state, KVM_CREATE_GUEST_MEMFD, &guest_memfd);
>       if (fd < 0) {
>           error_setg_errno(errp, errno, "Error creating KVM guest_memfd");

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