On 20/05/2021 11:24, Marc Zyngier wrote:

On Wed, 19 May 2021 15:09:23 +0100,
Steven Price [off-list ref] wrote:

quoted

On 17/05/2021 19:09, Marc Zyngier wrote:

quoted

On Mon, 17 May 2021 13:32:39 +0100,
Steven Price [off-list ref] wrote:

[...]>>>> +bytes (i.e. 1/16th of the corresponding size). Each byte
contains a single tag

quoted

quoted

quoted

+value. This matches the format of ``PTRACE_PEEKMTETAGS`` and
+``PTRACE_POKEMTETAGS``.
+
 5. The kvm_run structure
 ========================
 

@@ -6362,6 +6396,25 @@ default.
 
 See Documentation/x86/sgx/2.Kernel-internals.rst for more details.
 
+7.26 KVM_CAP_ARM_MTE
+--------------------
+
+:Architectures: arm64
+:Parameters: none
+
+This capability indicates that KVM (and the hardware) supports exposing the
+Memory Tagging Extensions (MTE) to the guest. It must also be enabled by the
+VMM before the guest will be granted access.
+
+When enabled the guest is able to access tags associated with any memory given
+to the guest. KVM will ensure that the pages are flagged ``PG_mte_tagged`` so
+that the tags are maintained during swap or hibernation of the host; however
+the VMM needs to manually save/restore the tags as appropriate if the VM is
+migrated.
+
+When enabled the VMM may make use of the ``KVM_ARM_MTE_COPY_TAGS`` ioctl to
+perform a bulk copy of tags to/from the guest.
+

Missing limitation to AArch64 guests.

As mentioned previously it's not technically limited to AArch64, but
I'll expand this to make it clear that MTE isn't usable from a AArch32 VCPU.

I believe the architecture is quite clear that it *is* limited to
AArch64. The clarification is welcome though.

I explained that badly. A system supporting MTE doesn't have to have all
CPUs running AArch64 - fairly obviously you can boot a 32 bit OS on a
system supporting AArch64.

Since the KVM capability is a VM capability it's not architecturally
inconsistent to enable it even if all your CPUs are running AArch32 (at
EL1 and lower) - just a bit pointless.

However, given your comment that a mixture of AArch32/AArch64 VCPUs is a
bug - we can fail creation of AArch32 VCPUs and I'll explicitly document
this is a AArch64 only feature.

Thanks,

Steve

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