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

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Message-ID: <3e9f2388-89fe-f762-8e59-03f4d70320fc@arm.com>
Date:   Thu, 20 May 2021 11:52:11 +0100
From:   Steven Price <steven.price@....com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Dave Martin <Dave.Martin@....com>,
        Mark Rutland <mark.rutland@....com>,
        Thomas Gleixner <tglx@...utronix.de>, qemu-devel@...gnu.org,
        Juan Quintela <quintela@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Richard Henderson <richard.henderson@...aro.org>,
        Peter Maydell <peter.maydell@...aro.org>,
        Haibo Xu <Haibo.Xu@....com>, Andrew Jones <drjones@...hat.com>
Subject: Re: [PATCH v12 8/8] KVM: arm64: Document MTE capability and ioctl

On 20/05/2021 11:24, Marc Zyngier wrote:
> On Wed, 19 May 2021 15:09:23 +0100,
> Steven Price <steven.price@....com> wrote:
>>
>> On 17/05/2021 19:09, Marc Zyngier wrote:
>>> On Mon, 17 May 2021 13:32:39 +0100,
>>> Steven Price <steven.price@....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