Subject: Re: [PATCH v8 0/6] Support writable CPU ID registers from userspace

From: Oliver Upton <[email-masked]>
Date: Tue, 16 May 2023 16:31:29 +0000

On Tue, May 16, 2023 at 02:11:30PM +0100, Marc Zyngier wrote:
> On Tue, 16 May 2023 12:55:14 +0100,
> Cornelia Huck <[email-masked]> wrote:
> > 
> > Do you have more concrete ideas for QEMU CPU models already? Asking
> > because I wanted to talk about this at KVM Forum, so collecting what
> > others would like to do seems like a good idea :)
> 
> I'm not being asked, but I'll share my thoughts anyway! ;-)
> 
> I don't think CPU models are necessarily the most important thing.
> Specially when you look at the diversity of the ecosystem (and even
> the same CPU can be configured in different ways at integration
> time). Case in point, Neoverse N1 which can have its I/D caches made
> coherent or not. And the guest really wants to know which one it is
> (you can only lie in one direction).
> 
> But being able to control the feature set exposed to the guest from
> userspace is a huge benefit in terms of migration.
> 
> Now, this is only half of the problem (and we're back to the CPU
> model): most of these CPUs have various degrees of brokenness. Most of
> the workarounds have to be implemented by the guest, and are keyed on
> the MIDR values. So somehow, you need to be able to expose *all* the
> possible MIDR values that a guest can observe in its lifetime.
> 
> I have a vague prototype for that that I'd need to dust off and
> finish, because that's also needed for this very silly construct
> called big-little...

And the third half of the problem is all of the other IP bits that get
strung together into an SOC :) Errata that live beyond the CPU can
become guest-visible (interconnect for example) and that becomes a bit
difficult to express to the guest OS. So, beyond something like a
big-little VM where the rest of the IP should be shared, I'm a bit
fearful of migrating a VM cross-system.

But hey, userspace is in the drivers seat and it can do as it pleases.

Hopefully we wouldn't need a KVM-specific PV interface for MIDR
enumeration. Perhaps the errata management spec could be expanded to
describe a set of CPU implementations and associated errata...

-- 
Thanks,
Oliver

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