linux-kernel - Re: [PATCH] KVM: arm64: Constrain the host to the maximum shared SVE VL with pKVM

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Message-ID: <c504578e-a00a-4f8b-a7bd-329fac45aa5f@sirena.org.uk>
Date: Wed, 11 Sep 2024 14:33:09 +0100
From: Mark Brown <broonie@...nel.org>
To: Fuad Tabba <tabba@...gle.com>
Cc: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>,
	James Morse <james.morse@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Dave Martin <Dave.Martin@....com>,
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: arm64: Constrain the host to the maximum shared SVE
 VL with pKVM

On Wed, Sep 11, 2024 at 01:02:16PM +0100, Fuad Tabba wrote:
> On Tue, 10 Sept 2024 at 21:26, Mark Brown <broonie@...nel.org> wrote:

> The crash is triggered by arch/arm64/kernel/fpsimd.c:487
> (fpsimd_save_user_state(): if (WARN_ON(sve_get_vl() != vl)) ...) ,
> which ends up issuing a SIGKILL.

> The easiest way to consistently reproduce the crash is with a host and
> a guest both running the sve stress test (the one in the arm64
> selftests).

Hrm, I'm not able to reproduce that...

> I think the issue is that this patch doesn't cover all the cases that
> are setting or assuming ZCR_ELx_LEN_MASK as the vector length.

We can't ever assume that VL is ZCR_ELx_LEN_MASK since that might not be
physically supported, and hopefully for a symmetric system the number
read back from RDVL should end up being the same.  Though trying to make
the change you suggested *is* triggering something which is fun.

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