Re: [RFC PATCH v1 05/37] KVM: guest_memfd: Wire up kvm_get_memory_attributes() to per-gmem attributes

[Alexey Kardashevskiy <aik@amd.com>]

On 18/10/25 07:11, Ackerley Tng wrote:
> From: Sean Christopherson <seanjc@google.com>
> 
> 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 <seanjc@google.com>
> Co-developed-by: Ackerley Tng <ackerleytng@google.com>
> Signed-off-by: Ackerley Tng <ackerleytng@google.com>
> ---
>   include/linux/kvm_host.h |  2 ++
>   virt/kvm/guest_memfd.c   | 29 +++++++++++++++++++++++++++++
>   virt/kvm/kvm_main.c      |  3 +++
>   3 files changed, 34 insertions(+)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 512febf47c265..b8418cc5851f1 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -2543,6 +2543,8 @@ 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 26cec833766c3..f62facc3ab776 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -518,6 +518,35 @@ 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);
> +
> +	/*
> +	 * 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 false;
> +
> +	/*
> +	 * Don't take the filemap invalidation lock, as temporarily acquiring
> +	 * that lock wouldn't provide any meaningful protection.  The caller
> +	 * _must_ protect consumption of private vs. shared by checking
> +	 * mmu_invalidate_retry_gfn() under mmu_lock.
> +	 */
> +	guard(rcu)();
> +
> +	return kvm_gmem_get_attributes(file_inode(file),
> +				       kvm_gmem_get_index(slot, gfn));
> +}
> +EXPORT_SYMBOL_GPL(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 6c29770dfa7c8..c73ebdb73070e 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2660,6 +2660,9 @@ 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);


I am trying to make it work with TEE-IO where fd of VFIO MMIO is a dmabuf fd while the rest (guest RAM) is gmemfd. The above suggests that if there is gmemfd - then the memory attributes are handled by gmemfd which is... expected?

The problem at hand is that kvm_mmu_faultin_pfn() fails at "if (fault->is_private != kvm_mem_is_private(kvm, fault->gfn))" and marking MMIO as private using kvm_vm_ioctl_set_mem_attributes() does not work as kvm_gmem_get_memory_attributes() fails on dmabuf fds.

I worked around this like below but wonder what is the proper way? Thanks,


@@ -768,13 +768,13 @@ unsigned long kvm_gmem_get_memory_attributes(struct kvm *kvm, gfn_t gfn)
  	 */
  	if (!slot)
  		return 0;
  
  	CLASS(gmem_get_file, file)(slot);
  	if (!file)
-		return false;
+		return kvm_get_vm_memory_attributes(kvm, gfn);
  
  	/*
  	 * Don't take the filemap invalidation lock, as temporarily acquiring
  	 * that lock wouldn't provide any meaningful protection.  The caller
  	 * _must_ protect consumption of private vs. shared by checking
  	 * mmu_invalidate_retry_gfn() under mmu_lock.



-- 
Alexey