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

From: Marc Zyngier <maz@kernel.org>
To: Cornelia Huck <cohuck@redhat.com>
Cc: 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 14:11:30 +0100	[thread overview]
Message-ID: <867ct8mnel.wl-maz@kernel.org> (raw)
In-Reply-To: <87cz30h4nx.fsf@redhat.com>

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...

Thanks,

	M.

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

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