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

From: Marc Zyngier <maz@kernel.org>
To: Oliver Upton <oliver.upton@linux.dev>
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: Sun, 07 Jan 2024 11:37:41 +0000	[thread overview]
Message-ID: <86cyud9xx6.wl-maz@kernel.org> (raw)
In-Reply-To: <ZZmS32lXXlULIArw@linux.dev>

On Sat, 06 Jan 2024 17:50:23 +0000,
Oliver Upton <oliver.upton@linux.dev> wrote:
> 
> 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

Huh. That's full of... not a lot.

> > 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.

Indeed. The document you quoted acknowledges that there are issues
(one step up from the previous situation), but this is not enough to
independently develop a workaround that will survive the test of time.

For example, AC03_CPU_39 doesn't even list the failing encodings,
which may lead to real funnies if these encodings get used at some
point.

	M.

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