Re: [PATCH v6 1/5] KVM: arm64: Block cacheable PFNMAP mapping

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, Jun 06, 2025 at 11:11:56AM -0700, Sean Christopherson wrote:
> > @@ -1612,6 +1624,10 @@ 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))
> 
> I don't think this is correct, and there's a very real chance this will break
> existing setups.  PFNMAP memory isn't strictly device memory, and IIUC, KVM
> force DEVICE/NORMAL_NC based on kvm_is_device_pfn(), not based on VM_PFNMAP.

kvm_is_device_pfn() effecitvely means KVM can't use CMOs on that
PFN. It doesn't really mean anything more..

PFNMAP says the same thing, or at least from a mm perspective we don't
want drivers taking PFNMAP memory and then trying to guess if there
are struct pages/KVAs for it. PFNMAP memory is supposed to be fully
opaque.

Though that confusion seems to be a separate issue from this patch.

> 	if (kvm_is_device_pfn(pfn)) {
> 		/*
> 		 * If the page was identified as device early by looking at
> 		 * the VMA flags, vma_pagesize is already representing the
> 		 * largest quantity we can map.  If instead it was mapped
> 		 * 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()
> 		 * change things at the last minute.
> 		 */
> 		device = true;

"device" here is sort of a mis-nomer, it is really just trying to
setup the S2 so that CMOs are not going go to be done.

Calling it 'disable_cmo' would sure make this code clearer..

> @@ -1639,6 +1653,9 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>                 return -EFAULT;
>  
>         if (kvm_is_device_pfn(pfn)) {
> +               if (is_vma_cacheable)
> +                       return -EINVAL;
> +

eg

if (!kvm_can_use_cmo_pfn(pfn)) {
               if (is_vma_cacheable)
                       return -EINVAL;

>                  * If the page was identified as device early by looking at
>                  * the VMA flags, vma_pagesize is already representing the
> @@ -1722,6 +1739,11 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>                 prot |= KVM_PGTABLE_PROT_X;
>  
>         if (device) {
> +               if (is_vma_cacheable) {
> +                       ret = -EINVAL;
> +                       goto out;
> +               }

if (disable_cmo) {
               if (is_vma_cacheable)
                       return -EINVAL;

Makes alot more sense, right? If KVM can't do CMOs then it should not
attempt to use memory mapped into the VMA as cachable.

>                 if (vfio_allow_any_uc)
>                         prot |= KVM_PGTABLE_PROT_NORMAL_NC;
>                 else
> 

Regardless, this seems good for this patch at least.

Jason