linux-kernel - Re: [PATCH 3/6] kselftest/arm64: Corrupt P15 in the irritator when testing SSVE

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Message-ID: <36fb49f5-55d8-4890-a8d4-d2cba67a9185@sirena.org.uk>
Date: Wed, 6 Nov 2024 12:46:10 +0000
From: Mark Brown <broonie@...nel.org>
To: Mark Rutland <mark.rutland@....com>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Shuah Khan <shuah@...nel.org>,
	linux-arm-kernel@...ts.infradead.org,
	linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] kselftest/arm64: Corrupt P15 in the irritator when
 testing SSVE

On Wed, Nov 06, 2024 at 11:27:24AM +0000, Mark Rutland wrote:

> > +#ifndef SSVE
> >  	// And FFR
> >  	wrffr	p15.b
> >  #endif

> Both RDFFR and WRFFR are illegal in streaming mode unless FEAT_FA64 is
> implemented and enabled, so we cannot use DRFFR in the SSVE case.

Indeed, I'm surprised that managed to pass my testing...

> Is there a different instruction we can use?

We could just trash p0 with something like a load or add.

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