Re: [PATCH v2 5/7] KVM: arm64: MTE: Use stage-2 NoTagAccess memory attribute if supported

Peter Collingbourne
2025-01-13T20:48:08Z

On Mon, Jan 13, 2025 at 11:09 AM Catalin Marinas
<catalin.marinas@arm.com> wrote:
>
> On Sat, Jan 11, 2025 at 06:49:55PM +0530, Aneesh Kumar K.V wrote:
> > Catalin Marinas <catalin.marinas@arm.com> writes:
> > > On Fri, Jan 10, 2025 at 04:30:21PM +0530, Aneesh Kumar K.V (Arm) wrote:
> > >> Currently, the kernel won't start a guest if the MTE feature is enabled
> >
> > ...
> >
> > >> @@ -2152,7 +2162,8 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
> > >> if (!vma)
> > >> break;
> > >>
> > >> - if (kvm_has_mte(kvm) && !kvm_vma_mte_allowed(vma)) {
> > >> + if (kvm_has_mte(kvm) &&
> > >> + !kvm_has_mte_perm(kvm) && !kvm_vma_mte_allowed(vma)) {
> > >> ret = -EINVAL;
> > >> break;
> > >> }
> > >
> > > I don't think we should change this, or at least not how it's done above
> > > (Suzuki raised a related issue internally relaxing this for VM_PFNMAP).
> > >
> > > For standard memory slots, we want to reject them upfront rather than
> > > deferring to the fault handler. An example here is file mmap() passed as
> > > standard RAM to the VM. It's an unnecessary change in behaviour IMHO.
> > > I'd only relax this for VM_PFNMAP mappings further down in this
> > > function (and move the VM_PFNMAP check above; see Suzuki's internal
> > > patch, unless he posted it publicly already).
> >
> > But we want to handle memslots backed by pagecache pages for virtio-shm
> > here (virtiofs dax use case).
>
> Ah, I forgot about this use case. So with virtiofs DAX, does a host page
> cache page (host VMM mmap()) get mapped directly into the guest as a
> separate memory slot? In this case, the host vma would not have
> VM_MTE_ALLOWED set.
>
> > With MTE_PERM, we can essentially skip the
> > kvm_vma_mte_allowed(vma) check because we handle all types in the fault
> > handler.
>
> This was pretty much the early behaviour when we added KVM support for
> MTE, allow !VM_MTE_ALLOWED and trap them later. However, we disallowed
> VM_SHARED because of some non-trivial race. Commit d89585fbb308 ("KVM:
> arm64: unify the tests for VMAs in memslots when MTE is enabled")
> changed this behaviour and the VM_MTE_ALLOWED check happens upfront. A
> subsequent commit removed the VM_SHARED check.
>
> It's a minor ABI change but I'm trying to figure out why we needed this
> upfront check rather than simply dropping the VM_SHARED check. Adding
> Peter in case he remembers. I can't see any race if we simply skipped
> this check altogether, irrespective of FEAT_MTE_PERM.

I don't see a problem with removing the upfront check. The reason I
kept the check was IIRC just that there was already a check there and
its logic needed to be adjusted for my VM_SHARED changes.

Peter