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From: | David Hildenbrand |
Subject: | Re: [PATCH v10 2/6] arm64: kvm: Introduce MTE VM feature |
Date: | Wed, 31 Mar 2021 09:34:44 +0200 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0 |
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.
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.
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"
For user MTE, we get away with this as the MAP_ANONYMOUS requirement would filter it out while arch_add_memory() will ensure it's mapped as untagged in the linear map. See another recent fix for hotplugged memory: d15dfd31384b ("arm64: mte: Map hotplugged memory as Normal Tagged"). We needed to ensure that ZONE_DEVICE doesn't end up as tagged, only hoplugged memory. Both handled via arch_add_memory() in the arch code with ZONE_DEVICE starting at devm_memremap_pages().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.I think this should be fine since it's not a MAP_ANONYMOUS (we do allow MAP_SHARED to be tagged).
-- Thanks, David / dhildenb
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