linux-kernel - Re: [PATCH v4] arm64: cpuinfo: Expose MIDR_EL1 and REVIDR

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Message-ID: <20160614110154.GB14654@e104818-lin.cambridge.arm.com>
Date:	Tue, 14 Jun 2016 12:01:54 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Russell King - ARM Linux <linux@...linux.org.uk>
Cc:	Suzuki K Poulose <suzuki.poulose@....com>,
	Mark Rutland <mark.rutland@....com>,
	Steve Capper <steve.capper@...aro.org>, steve.capper@....com,
	will.deacon@....com, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4] arm64: cpuinfo: Expose MIDR_EL1 and REVIDR_EL1 to
 sysfs

On Mon, Jun 13, 2016 at 06:21:40PM +0100, Russell King - ARM Linux wrote:
> On Mon, Jun 13, 2016 at 06:08:09PM +0100, Suzuki K Poulose wrote:
> > From: Steve Capper <steve.capper@...aro.org>
> > 
> > It can be useful for JIT software to be aware of MIDR_EL1 and
> > REVIDR_EL1 to ascertain the presence of any core errata that could
> > affect codegen.
> > 
> > This patch exposes these registers through sysfs:
> > 
> > /sys/devices/system/cpu/cpu$ID/identification/midr
> > /sys/devices/system/cpu/cpu$ID/identification/revidr
> > 
> > where $ID is the cpu number. For big.LITTLE systems, one can have a
> > mixture of cores (e.g. Cortex A53 and Cortex A57), thus all CPUs need
> > to be enumerated.
> > 
> > If the kernel does not have valid information to populate these entries
> > with, an empty string is returned to userspace.
> 
> So there's no confusion, I've historically said no on 32-bit ARM to
> exposing the MIDR to userspace, and my position on that for 32-bit
> ARM has not changed.

It depends on what you mean by "exposing". You can already get this
information in two ways: /proc/cpuinfo parsing or KVM ioctls. Of course,
this information can be abused in ways the kernel people did not intend
but it's late to hide something already exposed.

What we don't have is REVIDR and that's something useful to a JIT in
deciding whether it needs to work around certain CPU errata (unless we
come up with another mechanism to inform user about hardware bugs).

-- 
Catalin