Re: [PATCH v8 09/46] KVM: guest_memfd: Introduce function to check GFN private/shared status

[Fuad Tabba]

On Fri, 19 Jun 2026 at 01:31, Ackerley Tng via B4 Relay
<devnull+ackerleytng.google.com@xxxxxxxxxx> wrote:
>
> From: Ackerley Tng <ackerleytng@xxxxxxxxxx>
>
> Introduce function for KVM to check the private/shared status of guest
> memory at a given GFN.
>
> This will be used in a later patch.
>
> Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
> Co-developed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Reviewed-by: Fuad Tabba <tabba@xxxxxxxxxx>

Cheers,
/fuad
> ---
>  include/linux/kvm_host.h |  2 ++
>  virt/kvm/guest_memfd.c   | 31 +++++++++++++++++++++++++++++++
>  2 files changed, 33 insertions(+)
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 3915da2a61778..27687fb9d5201 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -2575,6 +2575,8 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
>  #endif /* CONFIG_KVM_VM_MEMORY_ATTRIBUTES */
>
>  #ifdef CONFIG_KVM_GUEST_MEMFD
> +bool kvm_gmem_is_private(struct kvm *kvm, gfn_t gfn);
> +
>  int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
>                      gfn_t gfn, kvm_pfn_t *pfn, struct page **page,
>                      int *max_order);
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 8101f64e0366f..bca912db5be6e 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -510,6 +510,37 @@ static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
>         return 0;
>  }
>
> +bool kvm_gmem_is_private(struct kvm *kvm, gfn_t gfn)
> +{
> +       struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);
> +       struct inode *inode;
> +
> +       /*
> +        * If this gfn has no associated memslot, there's no chance of the gfn
> +        * being backed by private memory, since guest_memfd must be used for
> +        * private memory, and guest_memfd must be associated with some memslot.
> +        */
> +       if (!slot)
> +               return 0;
> +
> +       CLASS(gmem_get_file, file)(slot);
> +       if (!file)
> +               return 0;
> +
> +       inode = file_inode(file);
> +
> +       /*
> +        * Rely on the maple tree's internal RCU lock to ensure a
> +        * stable result. This result can become stale as soon as the
> +        * lock is dropped, so the caller _must_ still protect
> +        * consumption of private vs. shared by checking
> +        * mmu_invalidate_retry_gfn() under mmu_lock to serialize
> +        * against ongoing attribute updates.
> +        */
> +       return kvm_gmem_is_private_mem(inode, kvm_gmem_get_index(slot, gfn));
> +}
> +EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_gmem_is_private);
> +
>  static struct file_operations kvm_gmem_fops = {
>         .mmap           = kvm_gmem_mmap,
>         .open           = generic_file_open,
>
> --
> 2.55.0.rc0.738.g0c8ab3ebcc-goog
>
>