Re: [RFC PATCH] KVM: arm64: Workaround for Ampere AC03_CPU_36 (exception taken to an incorrect EL)

From: Oliver Upton <oliver.upton@linux.dev>
To: Marc Zyngier <maz@kernel.org>
Cc: Ilkka Koskinen <ilkka@os.amperecomputing.com>,
	James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Zenghui Yu <yuzenghui@huawei.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, Gavin Shan <gshan@redhat.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Raghavendra Rao Ananta <rananta@google.com>,
	D Scott Phillips <scott@os.amperecomputing.com>,
	linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
	linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH] KVM: arm64: Workaround for Ampere AC03_CPU_36 (exception taken to an incorrect EL)
Date: Sat, 6 Jan 2024 17:50:23 +0000	[thread overview]
Message-ID: <ZZmS32lXXlULIArw@linux.dev> (raw)
In-Reply-To: <87sf3au0bu.wl-maz@kernel.org>

On Sat, Jan 06, 2024 at 12:13:09PM +0000, Marc Zyngier wrote:

[...]

> > From 265cb193190c13c651d8e008d34d1d18505d4804 Mon Sep 17 00:00:00 2001
> > From: Oliver Upton <oliver.upton@linux.dev>
> > Date: Fri, 5 Jan 2024 23:18:14 +0000
> > Subject: [PATCH] KVM: arm64: Mitigate AmpereOne erratum AC03_CPU_36
> > 
> > The AmpereOne design suffers from an erratum where if an asynchronous
> > exception arrives while EL2 is modifying hypervisor exception controls
> > (i.e. HCR_EL2, SCTLR_EL2) the PE may take an invalid exception to
> > another EL.
> 
> Same questions about SCTLR_EL2 and the notion of "another EL".

I've got the same questions :) This is just a rewording of Ampere's
erratum description.

https://amperecomputing.com/customer-connect/products/AmpereOne-device-documentation

> Other than the passing comments, I'm OK with this patch. However, I am
> very worried that this is only the start of a very long game of
> whack-a-mole, because there is no actual documentation on what goes
> wrong.
> 
> For example, we have plenty of writes to SCTLR_EL2 (using the
> SCTLR_EL1 alias if running VHE) for MTE. Are any of those affected?
> 
> Short of having some solid handle on what is happening, I don't see
> how we can promise to support this system.

Completely agree. At least on the AmpereOne machines I have access to
this seems to do the trick, but that observation is no replacement for
full documentation.

-- 
Thanks,
Oliver

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