Re: [PATCH v9 5/6] KVM: arm64: Allow cacheable stage 2 mapping using VMA flags

[David Hildenbrand]

On 04.07.25 16:04, Jason Gunthorpe wrote:
> On Sat, Jun 21, 2025 at 04:21:10AM +0000, ankita@xxxxxxxxxx wrote:
> > --- a/arch/arm64/kvm/mmu.c
> > +++ b/arch/arm64/kvm/mmu.c
> > @@ -1681,18 +1681,53 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> >   	if (is_error_noslot_pfn(pfn))
> >   		return -EFAULT;
> >   
> > +	/*
> > +	 * Check if this is non-struct page memory PFN, and cannot support
> > +	 * CMOs. It could potentially be unsafe to access as cachable.
> > +	 */
> >   	if (vm_flags & (VM_PFNMAP | VM_MIXEDMAP) && !pfn_is_map_memory(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.
> > +		 * COW VM_PFNMAP is possible when doing a MAP_PRIVATE
> > +		 * /dev/mem mapping on systems that allow such mapping.
> > +		 * Reject such case.
> >   		 */
> > -		s2_force_noncacheable = true;
> > +		if (is_cow_mapping(vm_flags))
> > +			return -EINVAL;
>
> I still would like an explanation why we need to block this.
>
> COW PFNMAP is like MIXEDMAP, you end up with a VMA where there is a
> mixture of MMIO and normal pages. Arguably you are supposed to use
> vm_normal_page() not pfn_is_map_memory(), but that seems difficult for
> KVM.
>
> Given we exclude the cachable case with the pfn_is_map_memory() we
> know this is the non-struct page memory already, so why do we need to
> block the COW?
>
> I think the basic rule we are going for is that within the VMA the
> non-normal/special PTE have to follow the vma->vm_pgprot while the
> normal pages have to be cachable.
>
> So if we find a normal page (ie pfn_is_map_memory()) then we know it
> is cachable and s2_force_noncacheable = false. Otherwise we use the
> vm_pgprot to decide if the special PTE is cachable.
>
> David can you think of any reason to have this is_cow_mapping() test?

I think with that reasoning, it should be fine to drop it.

I think, the COW test made sense when we were talking about limiting it 
to VM_PFNMAP only and simplifying by dropping other checks. Then, it 
would have identified that something is certainly not "normal" memory.

--
Cheers,

David / dhildenb