linux-kernel - Re: [PATCH v3 0/2] MTE support for KVM guest

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Date:   Fri, 2 Oct 2020 16:38:11 +0100
From:   Steven Price <steven.price@....com>
To:     Andrew Jones <drjones@...hat.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Peter Maydell <peter.maydell@...aro.org>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Haibo Xu <Haibo.Xu@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        qemu-devel@...gnu.org, Dave Martin <Dave.Martin@....com>,
        Juan Quintela <quintela@...hat.com>,
        Richard Henderson <richard.henderson@...aro.org>,
        linux-kernel@...r.kernel.org, James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 0/2] MTE support for KVM guest

On 02/10/2020 15:36, Andrew Jones wrote:
> On Fri, Sep 25, 2020 at 10:36:05AM +0100, Steven Price wrote:
>> Version 3 of adding MTE support for KVM guests. See the previous (v2)
>> posting for background:
>>
>>   https://lore.kernel.org/r/20200904160018.29481-1-steven.price%40arm.com
>>
>> These patches add support to KVM to enable MTE within a guest. They are
>> based on Catalin's v9 MTE user-space support series[1] (currently in
>> next).
>>
>> Changes since v2:
>>
>>   * MTE is no longer a VCPU feature, instead it is a VM cap.
>>
>>   * Being a VM cap means easier probing (check for KVM_CAP_ARM_MTE).
>>
>>   * The cap must be set before any VCPUs are created, preventing any
>>     shenanigans where MTE is enabled for the guest after memory accesses
>>     have been performed.
>>
>> [1] https://lore.kernel.org/r/20200904103029.32083-1-catalin.marinas@arm.com
>>
>> Steven Price (2):
>>    arm64: kvm: Save/restore MTE registers
>>    arm64: kvm: Introduce MTE VCPU feature
>>
>>   arch/arm64/include/asm/kvm_emulate.h       |  3 +++
>>   arch/arm64/include/asm/kvm_host.h          |  7 +++++++
>>   arch/arm64/include/asm/sysreg.h            |  3 ++-
>>   arch/arm64/kvm/arm.c                       |  9 +++++++++
>>   arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h | 14 ++++++++++++++
>>   arch/arm64/kvm/mmu.c                       | 15 +++++++++++++++
>>   arch/arm64/kvm/sys_regs.c                  | 20 +++++++++++++++-----
>>   include/uapi/linux/kvm.h                   |  1 +
>>   8 files changed, 66 insertions(+), 6 deletions(-)
>>
>> -- 
>> 2.20.1
>>
>>
> 
> Hi Steven,
> 
> These patches look fine to me, but I'd prefer we have a working
> implementation in QEMU before we get too excited about the KVM
> bits. kvmtool isn't sufficient since it doesn't support migration
> (at least afaik). In the past we've implemented features in KVM
> that look fine, but then issues have been discovered when trying
> to enable them from QEMU, where we also support migration. This
> feature looks like there's risk of issues with the userspace side.
> Although these two patches would probably stay the same, even if
> userspace requires more support.

I agree kvmtool isn't a great test because it doesn't support migration. 
The support in this series is just the basic support for MTE in a guest 
and we'd need to wait for the QEMU implementation before deciding 
whether we need any extra support (e.g. kernel interfaces for 
reading/writing tags as discussed before).

However, I don't think there's much danger of the support in this series 
changing - so extra support can be added when/if it's needed, but I 
don't think we need to block these series on that - QEMU can just probe 
for whatever additional support it needs before enabling MTE in a guest. 
I plan to rebase/repost after -rc1 when the user space support has been 
merged.

Steve