linux-kernel - Re: [RFC PATCH 05/14] KVM: arm64: Always allow fixed cycle counter

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Message-ID: <Z0-HBsBgf6WB7x4R@linux.dev>
Date: Tue, 3 Dec 2024 14:32:38 -0800
From: Oliver Upton <oliver.upton@...ux.dev>
To: Marc Zyngier <maz@...nel.org>
Cc: kvmarm@...ts.linux.dev, Joey Gouly <joey.gouly@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>,
	Mingwei Zhang <mizhang@...gle.com>,
	Colton Lewis <coltonlewis@...gle.com>,
	Raghavendra Rao Ananta <rananta@...gle.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 05/14] KVM: arm64: Always allow fixed cycle counter

On Tue, Dec 03, 2024 at 09:32:10PM +0000, Marc Zyngier wrote:
> On Tue, 03 Dec 2024 19:32:11 +0000,
> Oliver Upton <oliver.upton@...ux.dev> wrote:
> > 
> > The fixed CPU cycle counter is mandatory for PMUv3, so it doesn't make a
> > lot of sense allowing userspace to filter it. Only apply the PMU event
> > filter to *programmed* event counters.
> 
> But that's a change in ABI, isn't it? We explicitly say in the
> documentation that the cycle counter can be filtered by specifying
> event 0x11.

Yeah... A bit of a dirty shortcut I took because I don't like the ABI,
but distaste isn't enough to break it :)

> More importantly, the current filtering works in terms of events, and
> not in terms of counters.
> 
> Instead of changing the ABI, how about simply not supporting filtering
> on such non-compliant HW? Surely that would simplify a few things.

Yeah, that sounds reasonable. Especially if we allow programmable event
counters where the event ID space doesn't match the architecture.

-- 
Thanks,
Oliver