linux-kernel - Re: [PATCH v2] KVM: arm64: Constrain the host to the maximum shared SVE VL with pKVM

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Message-ID: <17cb5e48-ca96-433a-a351-639dd339dfb7@sirena.org.uk>
Date: Thu, 12 Sep 2024 15:45:27 +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 v2] KVM: arm64: Constrain the host to the maximum shared
 SVE VL with pKVM

On Thu, Sep 12, 2024 at 03:21:43PM +0100, Fuad Tabba wrote:

> One part that you haven't changed is setting ZCR_EL2 during el2 setup:
> arch/arm64/include/asm/el2_setup.h: .Linit_sve_ : lines 290/291

> I guess at that point it's not straightforward to figure sve_max_vl.
> Is there a window after el2 setup where we might actually get the VL
> implied by ZCR_ELx_LEN_MASK, or would it always get set to
> sve_vq_from_vl(kvm_host_sve_max_vl) - 1 ?

Yeah, at that point we have no idea what any other cores might look like
and there's just generally a bootstrapping issue - we need to see the
actual maximum VLs for the PEs to work out what the maximum VL for the
system is so we can tell KVM what to enforce.

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