Re: [PATCH v15 13/21] KVM: x86/mmu: Handle guest page faults for guest_memfd with shared memory

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

On 7/18/2025 12:27 AM, Fuad Tabba wrote:
> From: Ackerley Tng <ackerleytng@google.com>
> 
> Update the KVM MMU fault handler to service guest page faults
> for memory slots backed by guest_memfd with mmap support. For such
> slots, the MMU must always fault in pages directly from guest_memfd,
> bypassing the host's userspace_addr.
> 
> This ensures that guest_memfd-backed memory is always handled through
> the guest_memfd specific faulting path, regardless of whether it's for
> private or non-private (shared) use cases.
> 
> Additionally, rename kvm_mmu_faultin_pfn_private() to
> kvm_mmu_faultin_pfn_gmem(), as this function is now used to fault in
> pages from guest_memfd for both private and non-private memory,
> accommodating the new use cases.
> 
> Co-developed-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Ackerley Tng <ackerleytng@google.com>
> Co-developed-by: Fuad Tabba <tabba@google.com>
> Signed-off-by: Fuad Tabba <tabba@google.com>

Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>


Note to myself:

After this patch, it looks possible that 
kvm_mmu_prepare_memory_fault_exit() in kvm_mmu_faultin_pfn_gmem() might 
be triggered for guest_memfd with mmap support, though I'm not sure if 
there is real case to trigger it.

This requires some change in QEMU when it adds support for guest_memfd 
mmap support, since current QEMU handles KVM_EXIT_MEMORY_FAULT by always 
converting the memory attribute.

> ---
>   arch/x86/kvm/mmu/mmu.c | 13 +++++++++----
>   1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 94be15cde6da..ad5f337b496c 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4511,8 +4511,8 @@ static void kvm_mmu_finish_page_fault(struct kvm_vcpu *vcpu,
>   				 r == RET_PF_RETRY, fault->map_writable);
>   }
>   
> -static int kvm_mmu_faultin_pfn_private(struct kvm_vcpu *vcpu,
> -				       struct kvm_page_fault *fault)
> +static int kvm_mmu_faultin_pfn_gmem(struct kvm_vcpu *vcpu,
> +				    struct kvm_page_fault *fault)
>   {
>   	int max_order, r;
>   
> @@ -4536,13 +4536,18 @@ static int kvm_mmu_faultin_pfn_private(struct kvm_vcpu *vcpu,
>   	return RET_PF_CONTINUE;
>   }
>   
> +static bool fault_from_gmem(struct kvm_page_fault *fault)
> +{
> +	return fault->is_private || kvm_memslot_is_gmem_only(fault->slot);
> +}
> +
>   static int __kvm_mmu_faultin_pfn(struct kvm_vcpu *vcpu,
>   				 struct kvm_page_fault *fault)
>   {
>   	unsigned int foll = fault->write ? FOLL_WRITE : 0;
>   
> -	if (fault->is_private)
> -		return kvm_mmu_faultin_pfn_private(vcpu, fault);
> +	if (fault_from_gmem(fault))
> +		return kvm_mmu_faultin_pfn_gmem(vcpu, fault);
>   
>   	foll |= FOLL_NOWAIT;
>   	fault->pfn = __kvm_faultin_pfn(fault->slot, fault->gfn, foll,