Re: [PATCH v2 02/12] KVM: arm64: guest_memfd: Check mmap support when preparing memslot

[Suzuki K Poulose <suzuki.poulose@arm.com>]

On 11/11/2026 14:57, Steven Price wrote:
> Check the guest_memfd backing file when deciding whether an arm64
> memslot has mappable memory rather than checking whether the slot is
> gmem only. A public helper function is added to be able to reuse the
> static kvm_gmem_supports_mmap() function.
> 

Please could you explain why we need this ? Why not rely on
kvm_memslot_is_gmem_only(), which already tells kvm_gmem_supports_mmap() 
? With inplace conversion support gmem supports mmap.



> Signed-off-by: Steven Price <steven.price@arm.com>
> 
> ---
> Changes since v1:
> - New patch in v2. Check the guest_memfd backing file rather than the
>    memslot flag when deciding whether arm64 can map the backing memory.
> ---
>   arch/arm64/kvm/mmu.c     | 7 ++++---
>   include/linux/kvm_host.h | 1 +
>   virt/kvm/guest_memfd.c   | 6 ++++++
>   3 files changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 1db10d8d6db7..a146f6cde3d0 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -2839,10 +2839,11 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
>   		return -EFAULT;
>   
>   	/*
> -	 * Only support guest_memfd backed memslots with mappable memory, since
> -	 * there aren't any CoCo VMs that support only private memory on arm64.
> +	 * Protected VMs currently require guest_memfd backed memslots with
> +	 * mappable memory to support shared mappings.
>   	 */
> -	if (kvm_slot_has_gmem(new) && !kvm_memslot_is_gmem_only(new))
> +	if (kvm_arch_has_private_mem(kvm) && kvm_slot_has_gmem(new) &&
> +	    !kvm_gmem_slot_supports_mmap(new))
>   		return -EINVAL;
>   
>   	/* guest_memfd is incompatible with MTE. */
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index e7fc94e2910a..f65728a713ec 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -2614,6 +2614,7 @@ static inline bool kvm_is_private_gfn(struct kvm *kvm, gfn_t gfn)
>   
>   #ifdef CONFIG_KVM_GUEST_MEMFD
>   bool kvm_gmem_is_private_gfn(struct kvm *kvm, gfn_t gfn);
> +bool kvm_gmem_slot_supports_mmap(const struct kvm_memory_slot *slot);
>   bool kvm_arch_supports_gmem_init_shared(struct kvm *kvm);
>   
>   static inline u64 kvm_gmem_get_supported_flags(struct kvm *kvm)
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 8c97a69b16dc..3dd8d227bebb 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -442,6 +442,12 @@ static bool kvm_gmem_supports_mmap(struct inode *inode)
>   	return GMEM_I(inode)->flags & GUEST_MEMFD_FLAG_MMAP;
>   }
>   
> +bool kvm_gmem_slot_supports_mmap(const struct kvm_memory_slot *slot)
> +{
> +	/* The guest_memfd file is pinned while preparing a new memslot. */

We should gate this on kvm_slot_has_gmem() and how do we ensure that the
call comes after it is prepared ?

> +	return kvm_gmem_supports_mmap(file_inode(slot->gmem.file));
> +}
> +
>   static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf)
>   {
>   	struct inode *inode = file_inode(vmf->vma->vm_file);


Cheers
Suzuki