Re: [PATCH v3 6/7] KVM: arm64: permit all VM_MTE_ALLOWED mappings with MTE enabled

On 02/09/2022 14:45, Catalin Marinas wrote:

On Wed, Aug 10, 2022 at 12:30:32PM -0700, Peter Collingbourne wrote:

quoted

Certain VMMs such as crosvm have features (e.g. sandboxing) that depend
on being able to map guest memory as MAP_SHARED. The current restriction
on sharing MAP_SHARED pages with the guest is preventing the use of
those features with MTE. Now that the races between tasks concurrently
clearing tags on the same page have been fixed, remove this restriction.

Signed-off-by: Peter Collingbourne <redacted>
---
 arch/arm64/kvm/mmu.c | 8 --------
 1 file changed, 8 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index d54be80e31dd..fc65dc20655d 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c

@@ -1075,14 +1075,6 @@ static void sanitise_mte_tags(struct kvm *kvm, kvm_pfn_t pfn,
 
 static bool kvm_vma_mte_allowed(struct vm_area_struct *vma)
 {
-	/*
-	 * VM_SHARED mappings are not allowed with MTE to avoid races
-	 * when updating the PG_mte_tagged page flag, see
-	 * sanitise_mte_tags for more details.
-	 */
-	if (vma->vm_flags & VM_SHARED)
-		return false;

I think this is fine with the locking in place (BTW, it may be worth
mentioning in the commit message that it's a relaxation of the ABI). I'd
like Steven to have a look as well when he gets the time, in case we
missed anything on the KVM+MTE side.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

Looks fine to me, and thanks for doing the work: I was never very
pleased with the !VM_SHARED restriction, but I couldn't figure a good
way of getting the locking to work.

Reviewed-by: Steven Price <steven.price@arm.com>

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