Re: [PATCH v14 09/21] KVM: guest_memfd: Track guest_memfd mmap support in memslot

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 7/15/2025 5:33 PM, Fuad Tabba wrote:
> Add a new internal flag, KVM_MEMSLOT_GMEM_ONLY, to the top half of
> memslot->flags. This flag tracks when a guest_memfd-backed memory slot
> supports host userspace mmap operations. It's strictly for KVM's
> internal use.

I would expect some clarification of why naming it with 
KVM_MEMSLOT_GMEM_ONLY, not something like KVM_MEMSLOT_GMEM_MMAP_ENABLED

There was a patch to check the userspace_addr of the memslot refers to 
the same memory as guest memfd[1], but that patch was dropped. Without 
the background that when guest memfd is mmapable, userspace doesn't need 
to provide separate memory via userspace_addr, it's hard to understand 
and accept the name of GMEM_ONLY.

[1] https://lore.kernel.org/all/20250513163438.3942405-9-tabba@google.com/

> This optimization avoids repeatedly checking the underlying guest_memfd
> file for mmap support, which would otherwise require taking and
> releasing a reference on the file for each check. By caching this
> information directly in the memslot, we reduce overhead and simplify the
> logic involved in handling guest_memfd-backed pages for host mappings.
> 
> Reviewed-by: Gavin Shan <gshan@redhat.com>
> Reviewed-by: Shivank Garg <shivankg@amd.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> Suggested-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
>   include/linux/kvm_host.h | 11 ++++++++++-
>   virt/kvm/guest_memfd.c   |  2 ++
>   2 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 9ac21985f3b5..d2218ec57ceb 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -54,7 +54,8 @@
>    * used in kvm, other bits are visible for userspace which are defined in
>    * include/uapi/linux/kvm.h.
>    */
> -#define KVM_MEMSLOT_INVALID	(1UL << 16)
> +#define KVM_MEMSLOT_INVALID			(1UL << 16)
> +#define KVM_MEMSLOT_GMEM_ONLY			(1UL << 17)
>   
>   /*
>    * Bit 63 of the memslot generation number is an "update in-progress flag",
> @@ -2536,6 +2537,14 @@ static inline void kvm_prepare_memory_fault_exit(struct kvm_vcpu *vcpu,
>   		vcpu->run->memory_fault.flags |= KVM_MEMORY_EXIT_FLAG_PRIVATE;
>   }
>   
> +static inline bool kvm_memslot_is_gmem_only(const struct kvm_memory_slot *slot)
> +{
> +	if (!IS_ENABLED(CONFIG_KVM_GMEM_SUPPORTS_MMAP))
> +		return false;
> +
> +	return slot->flags & KVM_MEMSLOT_GMEM_ONLY;
> +}
> +
>   #ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
>   static inline unsigned long kvm_get_memory_attributes(struct kvm *kvm, gfn_t gfn)
>   {
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 07a4b165471d..2b00f8796a15 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -592,6 +592,8 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
>   	 */
>   	WRITE_ONCE(slot->gmem.file, file);
>   	slot->gmem.pgoff = start;
> +	if (kvm_gmem_supports_mmap(inode))
> +		slot->flags |= KVM_MEMSLOT_GMEM_ONLY;
>   
>   	xa_store_range(&gmem->bindings, start, end - 1, slot, GFP_KERNEL);
>   	filemap_invalidate_unlock(inode->i_mapping);