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

[Shameerali Kolothum Thodi]

> -----Original Message-----
> From: Marc Zyngier [mailto:maz at kernel.org]
> Sent: 16 May 2023 14:12
> To: Cornelia Huck <cohuck at redhat.com>
> Cc: Shameerali Kolothum Thodi <shameerali.kolothum.thodi at huawei.com>;
> Jing Zhang <jingzhangos at google.com>; KVM <kvm at vger.kernel.org>;
> KVMARM <kvmarm at lists.linux.dev>; ARMLinux
> <linux-arm-kernel at lists.infradead.org>; Oliver Upton <oupton at google.com>;
> Will Deacon <will at kernel.org>; Paolo Bonzini <pbonzini at redhat.com>;
> James Morse <james.morse at arm.com>; Alexandru Elisei
> <alexandru.elisei at arm.com>; Suzuki K Poulose <suzuki.poulose at arm.com>;
> Fuad Tabba <tabba at google.com>; Reiji Watanabe <reijiw at google.com>;
> Raghavendra Rao Ananta <rananta at google.com>
> Subject: Re: [PATCH v8 0/6] Support writable CPU ID registers from
> userspace
> 
> On Tue, 16 May 2023 12:55:14 +0100,
> Cornelia Huck <cohuck at redhat.com> 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.

Yes, this is what we also need and was thinking of adding a named CPU with
common min feature set exposed to Guest. There were some previous
attempts to add the basic support in Qemu here,

https://lists.gnu.org/archive/html/qemu-devel/2020-11/msg00087.html

> 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.

Ok. This will be a problem and I am not sure this has an impact on our 
platforms or not.

Thanks,
Shameer

> 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...
> 
> Thanks,
> 
> 	M.
> 
> --
> Without deviation from the norm, progress is not possible.