Re: [PATCH v11 03/40] arm64: cpufeature: Always specify and use a field width for capabilities

From: Marc Zyngier <maz@kernel.org>
To: Qian Cai <quic_qiancai@quicinc.com>
Cc: Mark Brown <broonie@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Shuah Khan <skhan@linuxfoundation.org>,
	"Shuah\ Khan" <shuah@kernel.org>,
	Basant Kumar Dwivedi <Basant.KumarDwivedi@arm.com>,
	Luis Machado <luis.machado@arm.com>,
	Szabolcs Nagy <szabolcs.nagy@arm.com>,
	<linux-arm-kernel@lists.infradead.org>,
	<linux-kselftest@vger.kernel.org>,
	Alan Hayward <alan.hayward@arm.com>,
	<kvmarm@lists.cs.columbia.edu>,
	"Salil\ Akerkar" <Salil.Akerkar@arm.com>
Subject: Re: [PATCH v11 03/40] arm64: cpufeature: Always specify and use a field width for capabilities
Date: Wed, 02 Mar 2022 10:12:09 +0000	[thread overview]
Message-ID: <878rts3bpi.wl-maz@kernel.org> (raw)
In-Reply-To: <Yh6kqaR9FQXg8Q4w@qian>

On Tue, 01 Mar 2022 22:56:41 +0000,
Qian Cai <quic_qiancai@quicinc.com> wrote:
> 
> On Mon, Feb 07, 2022 at 03:20:32PM +0000, Mark Brown wrote:
> > Since all the fields in the main ID registers are 4 bits wide we have up
> > until now not bothered specifying the width in the code. Since we now
> > wish to use this mechanism to enumerate features from the floating point
> > feature registers which do not follow this pattern add a width to the
> > table.  This means updating all the existing table entries but makes it
> > less likely that we run into issues in future due to implicitly assuming
> > a 4 bit width.
> > 
> > Signed-off-by: Mark Brown <broonie@kernel.org>
> 
> Do we leave this one alone on purpose?
> 
>                 .desc = "GIC system register CPU interface",
>                 .capability = ARM64_HAS_SYSREG_GIC_CPUIF,
>                 .type = ARM64_CPUCAP_STRICT_BOOT_CPU_FEATURE,
>                 .matches = has_useable_gicv3_cpuif,
>                 .sys_reg = SYS_ID_AA64PFR0_EL1,
>                 .field_pos = ID_AA64PFR0_GIC_SHIFT,
>                 .sign = FTR_UNSIGNED,
>                 .min_field_value = 1,
>
> Since width == 0, it will generate an undefined behavior.

I don't think that's on purpose, and we should definitely address
this. Maybe we should have a warning if we spot an occurrence of
.width being 0.

Thanks,

	M.

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

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