Re: [PATCH v10 2/6] arm64: kvm: Introduce MTE VM feature

[Catalin Marinas]

On Sat, Mar 27, 2021 at 03:23:24PM +0000, Catalin Marinas wrote:
> On Fri, Mar 12, 2021 at 03:18:58PM +0000, Steven Price wrote:
> > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > index 77cb2d28f2a4..b31b7a821f90 100644
> > --- a/arch/arm64/kvm/mmu.c
> > +++ b/arch/arm64/kvm/mmu.c
> > @@ -879,6 +879,22 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, 
> > phys_addr_t fault_ipa,
> >     if (vma_pagesize == PAGE_SIZE && !force_pte)
> >             vma_pagesize = transparent_hugepage_adjust(memslot, hva,
> >                                                        &pfn, &fault_ipa);
> > +
> > +   if (fault_status != FSC_PERM && kvm_has_mte(kvm) && pfn_valid(pfn)) {
> > +           /*
> > +            * VM will be able to see the page's tags, so we must ensure
> > +            * they have been initialised. if PG_mte_tagged is set, tags
> > +            * have already been initialised.
> > +            */
> > +           struct page *page = pfn_to_page(pfn);
> > +           unsigned long i, nr_pages = vma_pagesize >> PAGE_SHIFT;
> > +
> > +           for (i = 0; i < nr_pages; i++, page++) {
> > +                   if (!test_and_set_bit(PG_mte_tagged, &page->flags))
> > +                           mte_clear_page_tags(page_address(page));
> > +           }
> > +   }
> 
> This pfn_valid() check may be problematic. Following commit eeb0753ba27b
> ("arm64/mm: Fix pfn_valid() for ZONE_DEVICE based memory"), it returns
> true for ZONE_DEVICE memory but such memory is allowed not to support
> MTE.

Some more thinking, this should be safe as any ZONE_DEVICE would be
mapped as untagged memory in the kernel linear map. It could be slightly
inefficient if it unnecessarily tries to clear tags in ZONE_DEVICE,
untagged memory. Another overhead is pfn_valid() which will likely end
up calling memblock_is_map_memory().

However, the bigger issue is that Stage 2 cannot disable tagging for
Stage 1 unless the memory is Non-cacheable or Device at S2. Is there a
way to detect what gets mapped in the guest as Normal Cacheable memory
and make sure it's only early memory or hotplug but no ZONE_DEVICE (or
something else like on-chip memory)?  If we can't guarantee that all
Cacheable memory given to a guest supports tags, we should disable the
feature altogether.

> I now wonder if we can get a MAP_ANONYMOUS mapping of ZONE_DEVICE pfn
> even without virtualisation.

I haven't checked all the code paths but I don't think we can get a
MAP_ANONYMOUS mapping of ZONE_DEVICE memory as we normally need a file
descriptor.

-- 
Catalin