Re: [PATCH v10 2/6] arm64: kvm: Introduce MTE VM feature
From: David Hildenbrand
Date: Wed Mar 31 2021 - 05:33:52 EST
On 31.03.21 11:21, Catalin Marinas wrote:
On Wed, Mar 31, 2021 at 09:34:44AM +0200, David Hildenbrand wrote:
On 30.03.21 12:30, Catalin Marinas wrote:
On Mon, Mar 29, 2021 at 05:06:51PM +0100, Steven Price wrote:
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".
If you think "normal" == "normal System RAM", that's wrong; see below.
By "normal" I think both Steven and I meant the Normal Cacheable memory
attribute (another being the Device memory attribute).
That's the problem. With Anshuman's commit I mentioned above,
pfn_valid() returns true for ZONE_DEVICE mappings (e.g. persistent
memory, not talking about some I/O mapping that requires Device_nGnRE).
So kvm_is_device_pfn() is false for such memory and it may be mapped as
Normal but it is not guaranteed to support tagging.
pfn_valid() means "there is a struct page"; if you do pfn_to_page() and
touch the page, you won't fault. So Anshuman's commit is correct.
I agree.
pfn_to_online_page() means, "there is a struct page and it's system RAM
that's in use; the memmap has a sane content"
Does pfn_to_online_page() returns a valid struct page pointer for
ZONE_DEVICE pages? IIUC, these are not guaranteed to be system RAM, for
some definition of system RAM (I assume NVDIMM != system RAM). For
example, pmem_attach_disk() calls devm_memremap_pages() and this would
use the Normal Cacheable memory attribute without necessarily being
system RAM.
No, not for ZONE_DEVICE.
However, if you expose PMEM via dax/kmem as System RAM to the system (->
add_memory_driver_managed()), then PMEM (managed via ZONE_NOMRAL or
ZONE_MOVABLE) would work with pfn_to_online_page() -- as the system
thinks it's "ordinary system RAM" and the memory is managed by the buddy.
So if pfn_valid() is not equivalent to system RAM, we have a potential
issue with MTE. Even if "system RAM" includes NVDIMMs, we still have
this issue and we may need a new term to describe MTE-safe memory. In
the kernel we assume MTE-safe all pages that can be mapped as
MAP_ANONYMOUS and I don't think these include ZONE_DEVICE pages.
Thanks.
--
Thanks,
David / dhildenb