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

From: Oliver Upton <oliver.upton@linux.dev>
To: Marc Zyngier <maz@kernel.org>
Cc: Cornelia Huck <cohuck@redhat.com>,
	Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>,
	Jing Zhang <jingzhangos@google.com>, KVM <kvm@vger.kernel.org>,
	KVMARM <kvmarm@lists.linux.dev>,
	ARMLinux <linux-arm-kernel@lists.infradead.org>,
	Oliver Upton <oupton@google.com>, Will Deacon <will@kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	James Morse <james.morse@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Fuad Tabba <tabba@google.com>, Reiji Watanabe <reijiw@google.com>,
	Raghavendra Rao Ananta <rananta@google.com>
Subject: Re: [PATCH v8 0/6] Support writable CPU ID registers from userspace
Date: Tue, 16 May 2023 16:31:29 +0000	[thread overview]
Message-ID: <ZGOv4ZA9qxcC5wKv@linux.dev> (raw)
In-Reply-To: <867ct8mnel.wl-maz@kernel.org>

On Tue, May 16, 2023 at 02:11:30PM +0100, Marc Zyngier wrote:
> On Tue, 16 May 2023 12:55:14 +0100,
> Cornelia Huck <cohuck@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.
> 
> 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