On 28/03/2021 13:21, Catalin Marinas wrote:
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.
In stage 2 I believe we only have two types of mapping - 'normal' or
DEVICE_nGnRE (see stage2_map_set_prot_attr()). Filtering out the latter
is a case of checking the 'device' variable, and makes sense to avoid
the overhead you describe.
This should also guarantee that all stage-2 cacheable memory supports
tags, as kvm_is_device_pfn() is simply !pfn_valid(), and pfn_valid()
should only be true for memory that Linux considers "normal".
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.
I certainly hope this is the case - it's the weird corner cases of
device drivers that worry me. E.g. I know i915 has a "hidden" mmap
behind an ioctl (see i915_gem_mmap_ioctl(), although this case is fine -
it's MAP_SHARED). Mali's kbase did something similar in the past.
Steve
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