Re: [PATCH v12 4/8] arm64: kvm: Introduce MTE VM feature
From: Catalin Marinas <catalin.marinas@arm.com>
Date: 2021-05-20 11:56:30
Also in:
kvmarm, lkml, qemu-devel
On Mon, May 17, 2021 at 01:32:35PM +0100, Steven Price wrote:
quoted hunk ↗ jump to hunk
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c index c5d1f3c87dbd..8660f6a03f51 100644 --- a/arch/arm64/kvm/mmu.c +++ b/arch/arm64/kvm/mmu.c@@ -822,6 +822,31 @@ transparent_hugepage_adjust(struct kvm_memory_slot *memslot, return PAGE_SIZE; } +static int sanitise_mte_tags(struct kvm *kvm, unsigned long size, + kvm_pfn_t pfn) +{ + if (kvm_has_mte(kvm)) { + /* + * The page will be mapped in stage 2 as Normal Cacheable, so + * the VM will be able to see the page's tags and therefore + * they must be initialised first. If PG_mte_tagged is set, + * tags have already been initialised. + */ + unsigned long i, nr_pages = size >> PAGE_SHIFT; + struct page *page = pfn_to_online_page(pfn); + + if (!page) + return -EFAULT;
IIRC we ended up with pfn_to_online_page() to reject ZONE_DEVICE pages that may be mapped into a guest and we have no idea whether they support MTE. It may be worth adding a comment, otherwise, as Marc said, the page wouldn't disappear.
+
+ 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));We started the page->flags thread and ended up fixing it for the host set_pte_at() as per the first patch: https://lore.kernel.org/r/c3293d47-a5f2-ea4a-6730-f5cae26d8a7e@arm.com (local) Now, can we have a race between the stage 2 kvm_set_spte_gfn() and a stage 1 set_pte_at()? Only the latter takes a lock. Or between two kvm_set_spte_gfn() in different VMs? I think in the above thread we concluded that there's only a problem if the page is shared between multiple VMMs (MAP_SHARED). How realistic is this and what's the workaround? Either way, I think it's worth adding a comment here on the race on page->flags as it looks strange that here it's just a test_and_set_bit() while set_pte_at() uses a spinlock. -- Catalin _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel