On Thu, 7 May 2026 at 21:22, Ackerley Tng via B4 Relay
<[email protected]> wrote:
>
> From: Sean Christopherson <[email protected]>
>
> Implement kvm_gmem_get_memory_attributes() for guest_memfd to allow the KVM
> core and architecture code to query per-GFN memory attributes.
>
> kvm_gmem_get_memory_attributes() finds the memory slot for a given GFN and
> queries the guest_memfd file's to determine if the page is marked as
> private.
>
> If vm_memory_attributes is not enabled, there is no shared/private tracking
> at the VM level. Install the guest_memfd implementation as long as
> guest_memfd is enabled to give guest_memfd a chance to respond on
> attributes.
>
> guest_memfd should look up attributes regardless of whether this memslot is
> gmem-only since attributes are now tracked by gmem regardless of whether
> mmap() is enabled.
>
> Signed-off-by: Sean Christopherson <[email protected]>
> Co-developed-by: Ackerley Tng <[email protected]>
> Signed-off-by: Ackerley Tng <[email protected]>
> ---
> include/linux/kvm_host.h | 2 ++
> virt/kvm/guest_memfd.c | 31 +++++++++++++++++++++++++++++++
> virt/kvm/kvm_main.c | 3 +++
> 3 files changed, 36 insertions(+)
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index c5ba2cb34e45c..28a54298d27db 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -2557,6 +2557,8 @@ bool kvm_arch_post_set_memory_attributes(struct kvm
> *kvm,
> struct kvm_gfn_range *range);
> #endif /* CONFIG_KVM_VM_MEMORY_ATTRIBUTES */
>
> +unsigned long kvm_gmem_get_memory_attributes(struct kvm *kvm, gfn_t gfn);
> +
> #ifdef CONFIG_KVM_GUEST_MEMFD
> int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
> gfn_t gfn, kvm_pfn_t *pfn, struct page **page,
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 5011d38820d0d..f055e058a3f28 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -509,6 +509,37 @@ static int kvm_gmem_mmap(struct file *file, struct
> vm_area_struct *vma)
> return 0;
> }
>
> +unsigned long kvm_gmem_get_memory_attributes(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_get_attributes(inode, kvm_gmem_get_index(slot, gfn));
> +}
Doesn't this imply that all consumers of kvm_mem_is_private() should
validate the result using mmu_lock and the invalidation sequence?
sev_handle_rmp_fault() calls kvm_mem_is_private() without holding
mmu_lock and without any retry mechanism. Is that a problem?
Cheers,
/fuad
> +EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_gmem_get_memory_attributes);
> +
> static struct file_operations kvm_gmem_fops = {
> .mmap = kvm_gmem_mmap,
> .open = generic_file_open,
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index ee26f1d9b5fda..4139e903f756a 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2653,6 +2653,9 @@ static void kvm_init_memory_attributes(void)
> if (vm_memory_attributes)
> static_call_update(__kvm_get_memory_attributes,
> kvm_get_vm_memory_attributes);
> + else if (IS_ENABLED(CONFIG_KVM_GUEST_MEMFD))
> + static_call_update(__kvm_get_memory_attributes,
> + kvm_gmem_get_memory_attributes);
> else
> static_call_update(__kvm_get_memory_attributes,
> (void *)__static_call_return0);
>
> --
> 2.54.0.563.g4f69b47b94-goog
>
>
