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

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Message-ID: <a3f2f13e-be22-4a09-a8a6-5faef818defe@sirena.org.uk>
Date: Wed, 4 Sep 2024 16:48:57 +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:
> On Fri, Jul 05, 2024 at 06:18:50PM +0100, Mark Brown wrote:
> > On Fri, Jul 05, 2024 at 02:20:05PM +0100, Marc Zyngier wrote:
> > > Mark Brown <broonie@...nel.org> wrote:

> > > > As observed during review the pKVM support for saving host SVE state is
> > > > broken if an asymmetric system has VLs larger than the maximum shared
> > > > VL, fix this by discovering then using the maximum VL for allocations
> > > > and using RDVL during the save/restore process.

> > > I really don't see why we need such complexity here.

> The first patch is orthogonal cleanup, and the rest doesn't really add
> complexity IIUC.

...

> > Basically it's maintainability concerns, especially with the enumeration
> > code.

> I tend to agree here.

Did anyone have any further thoughts on this?  It's been roughly a
release cycle since I originally posted this, and there's been no
changes requested since -rc1 (which was itself just a rebase).

> 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's one for the pKVM people.

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