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

Oliver Upton
2025-02-24T22:23:26Z

Hi Sebastian,

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.

Thanks,
Oliver