linux-kernel - Re: [PATCH 0/3] KVM: arm64: Handle CCSIDR associativity mismatches

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Message-ID: <634e1d66-12f5-0b6c-7917-dc7982072711@daynix.com>
Date:   Sun, 11 Dec 2022 19:44:38 +0900
From:   Akihiko Odaki <akihiko.odaki@...nix.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Akihiko Odaki <akihiko.odaki@...il.com>,
        linux-kernel@...r.kernel.org, kvmarm@...ts.linux.dev,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        James Morse <james.morse@....com>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        asahi@...ts.linux.dev, Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Sven Peter <sven@...npeter.dev>,
        Hector Martin <marcan@...can.st>
Subject: Re: [PATCH 0/3] KVM: arm64: Handle CCSIDR associativity mismatches

On 2022/12/11 19:21, Marc Zyngier wrote:
> On Sun, 11 Dec 2022 05:25:31 +0000,
> Akihiko Odaki <akihiko.odaki@...nix.com> wrote:
>>
>> On 2022/12/04 23:57, Marc Zyngier wrote:
>>> On Fri, 02 Dec 2022 09:55:24 +0000,
>>> Akihiko Odaki <akihiko.odaki@...il.com> wrote:
>>>>
>>>> On 2022/12/02 18:40, Marc Zyngier wrote:
>>>>> On Fri, 02 Dec 2022 05:17:12 +0000,
>>>>> Akihiko Odaki <akihiko.odaki@...nix.com> wrote:
>>>>>>
>>>>>>>> On M2 MacBook Air, I have seen no other difference in standard ID
>>>>>>>> registers and CCSIDRs are exceptions. Perhaps Apple designed this way
>>>>>>>> so that macOS's Hypervisor can freely migrate vCPU, but I can't assure
>>>>>>>> that without more analysis. This is still enough to migrate vCPU
>>>>>>>> running Linux at least.
>>>>>>>
>>>>>>> I guess that MacOS hides more of the underlying HW than KVM does. And
>>>>>>> KVM definitely doesn't hide the MIDR_EL1 registers, which *are*
>>>>>>> different between the two clusters.
>>>>>>
>>>>>> It seems KVM stores a MIDR value of a CPU and reuse it as "invariant"
>>>>>> value for ioctls while it exposes the MIDR value each physical CPU
>>>>>> owns to vCPU.
>>>>>
>>>>> This only affects the VMM though, and not the guest which sees the
>>>>> MIDR of the CPU it runs on. The problem is that at short of pinning
>>>>> the vcpus, you don't know where they will run. So any value is fair
>>>>> game.
>>>>
>>>> Yes, my concern is that VMM can be confused if it sees something
>>>> different from what the guest on the vCPU sees.
>>>
>>> Well, this has been part of the ABI for about 10 years, since Rusty
>>> introduced this notion of invariant, so userspace is already working
>>> around it if that's an actual issue.
>>
>> In that case, I think it is better to document that the interface is
>> not working properly and deprecated.
> 
> This means nothing. Deprecating an API doesn't mean we don't support
> it and doesn't solve any issue for existing userspace.
> 
> I'd rather not change anything, TBH. Existing userspace already knows
> how to deal with this,
> 
>>
>>>
>>> This would be easily addressed though, and shouldn't result in any
>>> issue. The following should do the trick (only lightly tested on an
>>> M1).
>>
>> This can be problematic when restoring vcpu state saved with the old
>> kernel. A possible solution is to allow the userspace to overwrite
>> MIDR_EL1 as proposed for CCSIDR_EL1.
> 
> That would break most guests for obvious reasons. At best what can be
> done is to make the MIDR WI.

Making MIDR WI sounds good to me. Either keeping the current behavior or 
making it WI, the behavior is better to be documented, I think. The 
documentation obviously does not help running existing userspace code 
but will be helpful when writing new userspace code or understanding how 
existing userspace code works.

Regards,
Akihiko Odaki

> 
> 	M.
>