linux-kernel - Re: [PATCH v2 0/2] KVM: arm64: Support for Arm v8.8 memcpy instructions in KVM guests

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Message-ID: <6687f58c-0da9-0583-2dc1-2089f292b745@arm.com>
Date:   Thu, 28 Sep 2023 17:55:39 +0100
From:   Kristina Martsenko <kristina.martsenko@....com>
To:     Oliver Upton <oliver.upton@...ux.dev>
Cc:     kvmarm@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
        Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Zenghui Yu <yuzenghui@...wei.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Vladimir Murzin <vladimir.murzin@....com>,
        Colton Lewis <coltonlewis@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/2] KVM: arm64: Support for Arm v8.8 memcpy
 instructions in KVM guests

On 27/09/2023 07:00, Oliver Upton wrote:
> Hi Kristina,

Hi Oliver,

> 
> On Fri, Sep 22, 2023 at 12:25:06PM +0100, Kristina Martsenko wrote:
>> Hi,
>>
>> This is v2 of the series to allow using the new Arm memory copy instructions
>> in KVM guests. See v1 for more information [1].
> 
> 
> Thanks for sending out the series. I've been thinking about what the
> architecture says for MOPS, and I wonder if what's currently in the
> Arm ARM is clear enough for EL1 software to be written robustly.
> 
> While HCRX_EL2.MCE2 allows the hypervisor to intervene on MOPS
> exceptions from EL1, there's no such control for EL0. So when vCPU
> migration occurs EL1 could get an unexpected MOPS exception, even for a
> process that was pinned to a single (virtual) CPU implementation.
> 
> Additionally, the wording of I_NXHPS seems to suggest that EL2 handling
> of MOPS exceptions is only expected in certain circumstances where EL1 is
> incapable of handling an exception. Is the unwritten expectation then
> that EL1 software should tolerate 'unexpected' MOPS exceptions from EL1
> and EL0, even if EL1 did not migrate the PE context?
> 
> Perhaps I'm being pedantic, but I'd really like for there to be some
> documentation that suggests MOPS exceptions can happen due to context
> migration done by a higher EL as that is the only option in the context
> of virtualization.

That's a good point. This shouldn't affect Linux guests as Linux is
always able to handle a MOPS exception coming from EL0. But it would
affect any non-Linux guest that pins all its EL0 tasks and doesn't
implement a handler. It's not clear to me what the expectation for
guests is, I'll ask the architects to clarify and get back to you.

Thanks for the feedback!

Kristina