Re: [PATCH v2 1/1] KVM: arm64: Allow cacheable stage 2 mapping using VMA flags

[David Hildenbrand]

On 06.01.25 17:51, Jason Gunthorpe wrote:
> On Fri, Dec 20, 2024 at 04:42:35PM +0100, David Hildenbrand wrote:
> > On 18.11.24 14:19, ankita@xxxxxxxxxx wrote:
> > > From: Ankit Agrawal <ankita@xxxxxxxxxx>
> > >
> > > Currently KVM determines if a VMA is pointing at IO memory by checking
> > > pfn_is_map_memory(). However, the MM already gives us a way to tell what
> > > kind of memory it is by inspecting the VMA.
> >
> > Do you primarily care about VM_PFNMAP/VM_MIXEDMAP VMAs, or also other VMA
> > types?
>
> I think this is exclusively about allowing cachable memory inside a
> VM_PFNMAP VMA (created by VFIO) remain cachable inside the guest VM.

Thanks!

> > > This patch solves the problems where it is possible for the kernel to
> > > have VMAs pointing at cachable memory without causing
> > > pfn_is_map_memory() to be true, eg DAX memremap cases and CXL/pre-CXL
> > > devices. This memory is now properly marked as cachable in KVM.
> >
> > Does this only imply in worse performance, or does this also affect
> > correctness? I suspect performance is the problem, correct?
>
> Correctness. Things like atomics don't work on non-cachable mappings.

Hah! This needs to be highlighted in the patch description. And maybe 
this even implies Fixes: etc?

> > Maybe one could just reject such cases (if KVM PFN lookup code not
> > already rejects them, which might just be that case IIRC).
>
> At least VFIO enforces SHARED or it won't create the VMA.
>
> drivers/vfio/pci/vfio_pci_core.c:       if ((vma->vm_flags & VM_SHARED) == 0)

That makes a lot of sense for VFIO.

> This is pretty normal/essential for drivers..
>
> Are you suggesting the VMA flags should be inspected more?
> VM_SHARED/PFNMAP before allowing this?


I was wondering if we can safely assume that "device PFNs" can only 
exist in VM_PFNMAP mappings. Then we can avoid all that pfn_valid() / 
pfn_is_map_memory() stuff for "obviously not device" memory.

I tried to protoype, but have to give up for now; the code is too 
complicated to make quick progress :)

--
Cheers,

David / dhildenb