linux-kernel - Re: [PATCH v2 07/14] KVM: arm64: Use a cpucap to determine if system supports FEAT

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Message-ID: <86zfihramr.wl-maz@kernel.org>
Date: Wed, 19 Feb 2025 19:35:08 +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,
	Janne Grunau <j@...nau.net>
Subject: Re: [PATCH v2 07/14] KVM: arm64: Use a cpucap to determine if system supports FEAT_PMUv3

On Wed, 19 Feb 2025 19:22:56 +0000,
Oliver Upton <oliver.upton@...ux.dev> wrote:
> 
> On Wed, Feb 19, 2025 at 05:44:59PM +0000, Marc Zyngier wrote:
> > > +static bool has_pmuv3(const struct arm64_cpu_capabilities *entry, int scope)
> > > +{
> > > +	u64 dfr0 = read_sanitised_ftr_reg(SYS_ID_AA64DFR0_EL1);
> > > +	unsigned int pmuver;
> > > +
> > > +	pmuver = cpuid_feature_extract_unsigned_field(dfr0,
> > > +						      ID_AA64DFR0_EL1_PMUVer_SHIFT);
> > > +	if (pmuver == ID_AA64DFR0_EL1_PMUVer_IMP_DEF)
> > > +		return false;
> > > +
> > > +	return pmuver >= ID_AA64DFR0_EL1_PMUVer_IMP;
> > 
> > Given that PMUVer is a signed field, how about using
> > cpuid_feature_extract_signed_field() and do a signed comparison instead?
> 
> I'm happy to include a comment, but the PMUVer field is not signed. Any value
> other than 0xF is meant to be treated as an unsigned quantity.
> 
> DDI047L.a D24.1.3.2 is where this is coming from.

Duh, you're of course correct. Ignore me.

	M.

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