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

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Message-ID: <86iko4m5i4.wl-maz@kernel.org>
Date: Thu, 20 Mar 2025 09:10:59 +0000
From: Marc Zyngier <maz@...nel.org>
To: Akihiko Odaki <akihiko.odaki@...nix.com>
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 Thu, 20 Mar 2025 06:03:35 +0000,
Akihiko Odaki <akihiko.odaki@...nix.com> wrote:
> 
> On 2025/03/20 3:51, Oliver Upton wrote:
> > On Wed, Mar 19, 2025 at 06:38:38PM +0000, Marc Zyngier wrote:
> >> On Wed, 19 Mar 2025 11:51:21 +0000, Akihiko Odaki <akihiko.odaki@...nix.com> wrote:
> >>> 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.
> >> 
> >> How do you know that the affinity is "correct"? VCPU affinity can be
> >> changed at any time. I, for one, do not want my VMs to change
> >> behaviour because I let the vcpus bounce around as the scheduler sees
> >> fit.
> 
> Checking the affinity when picking the default PMU; the vCPU affinity
> is the only thing that rules the choice of the default PMU even now.
>
> Perhaps we may model the API as follows: introduce another "composite"
> PMU that works on any core but only exposes the cycle counter. Robust
> VMMs will choose it or one of hardware PMUs with
> KVM_ARM_VCPU_PMU_V3_SET_PMU. KVM will choose the default PMU according
> to the vCPU affinity at the point of KVM_ARM_VCPU_INIT otherwise. If
> the affinity is covered by one hardware PMU, that PMU will be chosen
> as the default. The "composite" PMU will be the default otherwise.

This makes no sense to me. A VCPU is always affine to a PMU, because
we do not support configurations where only some CPUs have a PMU. This
is an all-or-nothing situation.

More importantly, you keep suggesting the same "new default", and I
keep saying NO.

My position is clear: if you want a *new* behaviour, you *must* add a
new flag that the VMM explicitly provides to enable this CC-only PMU.
No change in default behaviour at all.

I'm not going to move from that.

	M.

-- 
Without deviation from the norm, progress is not possible.