linux-kernel - Re: [PATCH v4 3/4] KVM: arm64: Fix FFR offset calculation for pKVM host state save and restore

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Message-ID: <f1b357b6-b1c5-4d4e-a597-9fb2ff144627@sirena.org.uk>
Date: Fri, 5 Jul 2024 18:25:30 +0100
From: Mark Brown <broonie@...nel.org>
To: Marc Zyngier <maz@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>,
	James Morse <james.morse@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Fuad Tabba <tabba@...gle.com>, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, kvmarm@...ts.linux.dev
Subject: Re: [PATCH v4 3/4] KVM: arm64: Fix FFR offset calculation for pKVM
 host state save and restore

On Fri, Jul 05, 2024 at 02:27:01PM +0100, Marc Zyngier wrote:
> On Thu, 04 Jul 2024 18:28:18 +0100,
> Mark Brown <broonie@...nel.org> wrote:

> > When saving and restoring the SVE state for the host we configure the
> > hardware for the maximum VL it supports, but when calculating offsets in
> > memory we use the maximum usable VL for the host. Since these two values
> > may not be the same this may result in data corruption.  We can just
> > read the current VL from the hardware with an instruction so do that
> > instead of a saved value, we need to correct the value and this makes
> > the consistency obvious.

> Which value are we correcting?

The vector length used when laying out the storage, especially in the
case where the hardware VL is larger than the largest VL used by Linux.

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