Re: [PATCH v3 4/9] KVM: guest_memfd: add GUEST_MEMFD_FLAG_MIGRATABLE

On Wed, 2026-08-05 at 07:08 +0000, [email protected] wrote:
> Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
> - [Medium] Global capability check for KVM_CAP_GUEST_MEMFD_FLAGS falsely advertises GUEST_MEMFD_FLAG_MIGRATABLE on unsupported architectures.
> - [Low] Typo in API documentation for GUEST_MEMFD_FLAG_MIGRATABLE.
> --
> 
> --- Patch [4]: [PATCH v3 4/9] KVM: guest_memfd: add GUEST_MEMFD_FLAG_MIGRATABLE ---
> commit f12765f58cf3ad7c38abcf69c609570a66578e93
> Author: Shivank Garg <[email protected]>
> 
> KVM: guest_memfd: add GUEST_MEMFD_FLAG_MIGRATABLE
>     
> This patch adds architecture support checks and a new flag,
> GUEST_MEMFD_FLAG_MIGRATABLE, to allow the MM to migrate guest_memfd folios.
> It updates the mapping GFP mask and removes a redundant WARN_ON_ONCE.
> 
> > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> > index e3003a241d5b0..976921527ef9a 100644
> > --- a/Documentation/virt/kvm/api.rst
> > +++ b/Documentation/virt/kvm/api.rst
> > @@ -6461,6 +6461,9 @@ specified via KVM_CREATE_GUEST_MEMFD.  Currently defined flags:
> >                                 without INIT_SHARED will be marked private).
> >                                 Shared memory can be faulted into host userspace
> >                                 page tables. Private memory cannot.
> > + GUEST_MEMFD_FLAG_MIGRATABLE   Allow MM to migrate guest_memfd folios.
> > +                               Availability is architecture sepecific. Without
> 
> [Severity: Low]
> This isn't a bug, but there is a typo here (sepecific instead of specific).
> 

Ack. Will correct this.

> > +                               this flag, folios remain unmovable.
> >    ============================ ================================================
> 
> [ ... ]
> 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 47cb9eba113b1..7087b0b8f973f 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -14130,6 +14130,15 @@ bool kvm_arch_supports_gmem_init_shared(struct kvm *kvm)
> >  	return !kvm_arch_has_private_mem(kvm);
> >  }
> >  
> > +/*
> > + * Migration of guest_memfd with private memory is not supported yet
> > + * as this may require architecture-specific handling.
> > + */
> > +bool kvm_arch_supports_gmem_migration(struct kvm *kvm)
> > +{
> > +	return !kvm_arch_has_private_mem(kvm);
> > +}
> 
> [ ... ]
> 
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index ab8cfaec82d31..7b2f15b219705 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -731,6 +731,7 @@ static inline bool kvm_arch_has_private_mem(struct kvm *kvm)
> >  
> >  #ifdef CONFIG_KVM_GUEST_MEMFD
> >  bool kvm_arch_supports_gmem_init_shared(struct kvm *kvm);
> > +bool kvm_arch_supports_gmem_migration(struct kvm *kvm);
> >  
> >  static inline u64 kvm_gmem_get_supported_flags(struct kvm *kvm)
> >  {
> > @@ -738,6 +739,9 @@ static inline u64 kvm_gmem_get_supported_flags(struct kvm *kvm)
> >  
> >  	if (!kvm || kvm_arch_supports_gmem_init_shared(kvm))
> >  		flags |= GUEST_MEMFD_FLAG_INIT_SHARED;
> > +	if (IS_ENABLED(CONFIG_MIGRATION) &&
> > +	    (!kvm || kvm_arch_supports_gmem_migration(kvm)))
> > +		flags |= GUEST_MEMFD_FLAG_MIGRATABLE;
> 
> [Severity: Medium]
> When userspace queries KVM_CAP_GUEST_MEMFD_FLAGS on the global /dev/kvm
> file descriptor, kvm is NULL. Does the !kvm check here cause the API to
> unconditionally advertise GUEST_MEMFD_FLAG_MIGRATABLE for all architectures
> where CONFIG_MIGRATION is enabled?
> 
> If so, architectures that do not support migration (relying on the weak
> default returning false) would advertise support globally, but then return
> -EINVAL when userspace actually attempts to create a guest_memfd with this
> flag.
> 
> If we drop the !kvm bypass to fix the capability advertisement, would the
> x86 implementation of kvm_arch_supports_gmem_migration() shown above need an
> explicit check for a NULL kvm pointer before passing it to
> kvm_arch_has_private_mem()?
> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=4


So, IIUC, I can have an IS_ENABLED(CONFIG_MIGRATION) check to not advertise
the MIGRATE flag globally.

Thanks,
Shivank