Re: [PATCH v12 8/8] KVM: arm64: Document MTE capability and ioctl

[Steven Price]

On 20/05/2021 11:24, Marc Zyngier wrote:
> On Wed, 19 May 2021 15:09:23 +0100,
> Steven Price <steven.price@arm.com> wrote:
>>
>> On 17/05/2021 19:09, Marc Zyngier wrote:
>>> On Mon, 17 May 2021 13:32:39 +0100,
>>> Steven Price <steven.price@arm.com> wrote:
[...]>>>> +bytes (i.e. 1/16th of the corresponding size). Each byte
contains a single tag
>>>> +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