Re: [PATCH] arm64: Extend workaround for erratum 1024718 to all versions of Cortex-A55

From: Will Deacon <will@kernel.org>
To: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: catalin.marinas@arm.com,
	Kunihiko Hayashi <hayashi.kunihiko@socionext.com>,
	James Morse <james.morse@arm.com>,
	stable@vger.kernel.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] arm64: Extend workaround for erratum 1024718 to all versions of Cortex-A55
Date: Mon, 8 Feb 2021 12:29:00 +0000	[thread overview]
Message-ID: <20210208122859.GA25618@willie-the-truck> (raw)
In-Reply-To: <5a8ea892-98bb-e02e-cced-9ffa7e0bbda9@arm.com>

On Fri, Feb 05, 2021 at 05:41:05PM +0000, Suzuki K Poulose wrote:
> On 2/4/21 9:54 AM, Will Deacon wrote:
> > On Wed, Feb 03, 2021 at 11:00:57PM +0000, Suzuki K Poulose wrote:
> > > The erratum 1024718 affects Cortex-A55 r0p0 to r2p0. However
> > > we apply the work around for r0p0 - r1p0. Unfortunately this
> > > won't be fixed for the future revisions for the CPU. Thus
> > > extend the work around for all versions of A55, to cover
> > > for r2p0 and any future revisions.
> > > 
> > > Cc: stable@vger.kernel.org
> > > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > > Cc: Will Deacon <will@kernel.org>
> > > Cc: James Morse <james.morse@arm.com>
> > > Cc: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
> > > Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> > > ---
> > >   arch/arm64/kernel/cpufeature.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > > index e99eddec0a46..db400ca77427 100644
> > > --- a/arch/arm64/kernel/cpufeature.c
> > > +++ b/arch/arm64/kernel/cpufeature.c
> > > @@ -1455,7 +1455,7 @@ static bool cpu_has_broken_dbm(void)
> > >   	/* List of CPUs which have broken DBM support. */
> > >   	static const struct midr_range cpus[] = {
> > >   #ifdef CONFIG_ARM64_ERRATUM_1024718
> > > -		MIDR_RANGE(MIDR_CORTEX_A55, 0, 0, 1, 0),  // A55 r0p0 -r1p0
> > > +		MIDR_ALL_VERSIONS(MIDR_CORTEX_A55),
> > 
> > I think we have bigger problems with this erratum, since cpu_has_hw_af()
> > doesn't taken this erratum into account at all, meaning that
> > arch_faults_on_old_pte() will return the wrong value on any system with an
> > A55.
> 
> Please note that we enable HW_AF on these CPUs even with this erratum as
> they are not affected. It is only the DBM that we selectively disable. Thus
> the AF flag checks are still valid (See __cpu_setup in arch/arm64/mm/proc.S).
> Or am I miss something ?

Thanks, you're completely right! I'll queue this one then, and update the
Kconfig test at the same time.

Will

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