[PATCH] KVM: arm64: Drop mte_allowed check during memslot creation

[Aneesh Kumar K.V (Arm)]

Before commit d89585fbb308 ("KVM: arm64: unify the tests for VMAs in
memslots when MTE is enabled"), kvm_arch_prepare_memory_region() only
rejected a memory slot if VM_SHARED was set. This commit unified the
checking with user_mem_abort(), with slots being rejected if either
VM_MTE_ALLOWED is not set or VM_SHARED set. A subsequent commit
c911f0d46879 ("KVM: arm64: permit all VM_MTE_ALLOWED mappings with MTE
enabled") dropped the VM_SHARED check, so we ended up with memory slots
being rejected if VM_MTE_ALLOWED is not set. This wasn't the case before
the commit d89585fbb308. The rejection of the memory slot with VM_SHARED
set was done to avoid a race condition with the test/set of the
PG_mte_tagged flag. Before Commit d77e59a8fccd ("arm64: mte: Lock a page
for MTE tag initialization") the kernel avoided allowing MTE with shared
pages, thereby preventing two tasks sharing a page from setting up the
PG_mte_tagged flag racily.

Commit d77e59a8fccd ("arm64: mte: Lock a page for MTE tag
initialization") further updated the locking so that the kernel
allows VM_SHARED mapping with MTE. With this commit, we can enable
memslot creation with VM_SHARED VMA mapping.

This patch results in a minor tweak to the ABI. We now allow creating
memslots that don't have the VM_MTE_ALLOWED flag set. If the guest uses
such a memslot with Allocation Tags, the kernel will generate -EFAULT.
ie, instead of failing early, we now fail later during KVM_RUN.

This change is needed because, without it, users are not able to use MTE
with VFIO passthrough, as shown below (kvmtool VMM).

[  617.921030] vfio-pci 0000:01:00.0: resetting
[  618.024719] vfio-pci 0000:01:00.0: reset done
  Error: 0000:01:00.0: failed to register region with KVM
  Warning: [0abc:aced] Error activating emulation for BAR 0
  Error: 0000:01:00.0: failed to configure regions
  Warning: Failed init: vfio__init

  Fatal: Initialisation failed

Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@xxxxxxxxxx>
---
 arch/arm64/kvm/mmu.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 007dda958eab..610becd8574e 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -2146,11 +2146,6 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
 		if (!vma)
 			break;
=20
-		if (kvm_has_mte(kvm) && !kvm_vma_mte_allowed(vma)) {
-			ret =3D -EINVAL;
-			break;
-		}
-
 		if (vma->vm_flags & VM_PFNMAP) {
 			/* IO region dirty page logging not allowed */
 			if (new->flags & KVM_MEM_LOG_DIRTY_PAGES) {
--=20
2.43.0