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

[Peter Xu]

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 <pet...@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