linux-kernel - Re: [PATCH RFC] KVM: arm64: PMU: Use multiple host PMUs

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Message-ID: <cd7b4528-34a3-4d87-9711-acc2c2e6f6e1@daynix.com>
Date: Wed, 19 Mar 2025 20:51:21 +0900
From: Akihiko Odaki <akihiko.odaki@...nix.com>
To: Marc Zyngier <maz@...nel.org>
Cc: Oliver Upton <oliver.upton@...ux.dev>, Joey Gouly <joey.gouly@....com>,
 Suzuki K Poulose <suzuki.poulose@....com>, Zenghui Yu
 <yuzenghui@...wei.com>, Catalin Marinas <catalin.marinas@....com>,
 Will Deacon <will@...nel.org>, Kees Cook <kees@...nel.org>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>,
 linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
 linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
 devel@...nix.com
Subject: Re: [PATCH RFC] KVM: arm64: PMU: Use multiple host PMUs

On 2025/03/19 20:41, Marc Zyngier wrote:
> On Wed, 19 Mar 2025 11:26:18 +0000,
> Akihiko Odaki <akihiko.odaki@...nix.com> wrote:
>>
>> On 2025/03/19 20:07, Marc Zyngier wrote:
>>> On Wed, 19 Mar 2025 10:26:57 +0000,
>>>>
>>> But that'd be a new ABI, which again would require buy-in from
>>> userspace.  Maybe there is scope for an all CPUs, cycle-counter only
>>> PMUv3 exposed to the guest, but that cannot be set automatically, as
>>> we would otherwise regress existing setups.
>>>
>>> At this stage, and given that you need to change userspace, I'm not
>>> sure what the best course of action is.
>>
>> Having an explicit flag for the userspace is fine for QEMU, which I
>> care. It can flip the flag if and only if threads are not pinned to
>> one PMU and the machine is a new setup.
>>
>> I also wonder what regression you think setting it automatically causes.
> 
> The current behaviour is that if you don't specify anything other than
> creating a PMUv3 (without KVM_ARM_VCPU_PMU_V3_SET_PMU), you get *some*
> PMU, and userspace is responsible for running the vcpu on CPUs that
> will implement that PMU. When if does, all the counters, all the
> events are valid. If it doesn't, nothing counts, but the
> counters/events are still valid.
> 
> If you now add this flag automatically, the guest doesn't see the full
> PMU anymore. Only the cycle counter. That's the regression.

What about setting the flag automatically when a user fails to pin vCPUs 
to CPUs that are covered by one PMU? There would be no change if a user 
correctly pins vCPUs as it is. Otherwise, they will see a correct 
feature set advertised to the guest and the cycle counter working.

Regards,
Akihiko Odaki

> 
> Thanks,
> 
> 	M.
>