Re: [PATCH v6 4/5] KVM: arm64: Allow cacheable stage 2 mapping using VMA flags

[Sean Christopherson <seanjc@google.com>]

On Sat, May 24, 2025, ankita@nvidia.com wrote:
> @@ -1636,9 +1637,19 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  
>  	vfio_allow_any_uc = vma->vm_flags & VM_ALLOW_ANY_UNCACHED;
>  
> -	if ((vma->vm_flags & VM_PFNMAP) &&
> -	    !mapping_type_noncacheable(vma->vm_page_prot))
> -		return -EINVAL;
> +	if (vma->vm_flags & VM_PFNMAP) {
> +		/* Reject COW VM_PFNMAP */
> +		if (is_cow_mapping(vma->vm_flags))
> +			return -EINVAL;
> +
> +		/*
> +		 * If the VM_PFNMAP is set in VMA flags, do a KVM sanity
> +		 * check to see if pgprot mapping type is MT_NORMAL and a
> +		 * safely cacheable device memory.
> +		 */
> +		if (!mapping_type_noncacheable(vma->vm_page_prot))
> +			cacheable_devmem = true;
> +	}
>  
>  	/* Don't use the VMA after the unlock -- it may have vanished */
>  	vma = NULL;
> @@ -1671,10 +1682,13 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  		 * via __kvm_faultin_pfn(), vma_pagesize is set to PAGE_SIZE
>  		 * and must not be upgraded.
>  		 *
> -		 * In both cases, we don't let transparent_hugepage_adjust()
> +		 * Do not set device as the device memory is cacheable. Note
> +		 * that such mapping is safe as the KVM S2 will have the same
> +		 * Normal memory type as the VMA has in the S1.
>  		 * change things at the last minute.
>  		 */
> -		device = true;
> +		if (!cacheable_devmem)
> +			device = true;

I doubt this is correct.  "device" is used for more than just the memtype.  E.g.
hugepage adjustments, MTE, etc. all consult "device".  I.e. don't conflate device
with VM_PFNMAP.