linux-kernel - Re: [PATCH v4 0/4] KVM: arm64: Fix underallocation of storage for SVE state

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Message-ID: <69e0d4c2-b222-43d0-b95a-686fee465f79@sirena.org.uk>
Date: Mon, 8 Jul 2024 19:12:14 +0100
From: Mark Brown <broonie@...nel.org>
To: Dave Martin <Dave.Martin@....com>
Cc: Marc Zyngier <maz@...nel.org>,
	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 0/4] KVM: arm64: Fix underallocation of storage for
 SVE state

On Mon, Jul 08, 2024 at 04:30:30PM +0100, Dave Martin wrote:

> One thing confuses me:

> The host could never use over-max VLs except in non-preemptible kernel
> code, since code doing that would be non-migratable to other physical
> CPUs.  This is done to probe SVE only, and the extra bits in the vector
> registers are never used at all.

> Can't pKVM just hide the non symmetrically supported VLs using ZCR_EL2,
> just as regular KVM does for the guest?

> (I may be making bad assumptions about pKVM's relationship with the host
> kernel.)

That'd be another way to do it, constrain the VLs the host can set - I
assume there's something about how pKVM or hVHE does things that makes
problems for that.

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