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

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Message-ID: <87ldwwsbad.wl-maz@kernel.org>
Date: Tue, 03 Dec 2024 21:32:10 +0000
From: Marc Zyngier <maz@...nel.org>
To: Oliver Upton <oliver.upton@...ux.dev>
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, 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.

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.

Thanks,

	M.

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