linux-kernel - Re: [PATCH v3 0/4] KVM: arm64: writable MIDR/REVIDR

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Message-ID: <Z79j1ckWLx_Hwqct@linux.dev>
Date: Wed, 26 Feb 2025 10:56:21 -0800
From: Oliver Upton <oliver.upton@...ux.dev>
To: Sebastian Ott <sebott@...hat.com>
Cc: Marc Zyngier <maz@...nel.org>, Joey Gouly <joey.gouly@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
	Cornelia Huck <cohuck@...hat.com>,
	Eric Auger <eric.auger@...hat.com>,
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 0/4] KVM: arm64: writable MIDR/REVIDR

On Wed, Feb 26, 2025 at 05:47:53PM +0100, Sebastian Ott wrote:
> On Mon, 24 Feb 2025, Oliver Upton wrote:
> > On Tue, Feb 18, 2025 at 05:34:39PM +0100, Sebastian Ott wrote:
> > > Based on prior discussion [1] this series allows VMMs to change
> > > MIDR/REVIDR to enable migration between machines that differ in
> > > these registers. Since these are used for errata handling the
> > > errata management series [2] is a prerequisite for this one.
> > > 
> > > changes for V3:
> > > * handle VPIDR_EL2 as part of vcpu ctxt - thanks Oliver!
> > 
> > Thanks for respinning. While your changes are looking good, as I got
> > ready to apply this series I wound up peeling the onion a bit further
> > and have a few more concerns:
> > 
> > - Current KVM allows guests to read SMIDR_EL1 despite the fact that we
> >   do not support SME (this is part of TID1 traps)
> > 
> > - The "invariant" values that KVM presents to userspace originate from
> >   the boot CPU, not the CPU that resets the ID registers for a VM
> > 
> > - A VMM that wants to present big-little can do so on current KVM by
> >   affining vCPUs, but cannot with this series
> > 
> > All of this is to say, I think your series is going to collide with
> > the pre-existing pile of crap we have. I'm going to pick up these
> > changes and rework them so we can send a fix for #1 to stable trees and
> > (hopefully) avoid breaking the old "invariant" behavior.
> > 
> > I'll post what I have as soon as I test it, hopefully we can get this
> > shaped up for 6.15.
> 
> Sry, for the additional work I've caused. I gave what you have in next a
> spin and it looks good so far.
> Thank you very much!

Don't apologize -- it isn't your fault what we had already was a mess! :)

Thanks for kicking the tires.

Best,
Oliver