linux-kernel - Re: [RFC PATCH v1 0/2] KVM: arm64: Making BT Field in ID_AA64PFR1

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Date: Tue, 11 Jun 2024 22:30:51 -0700
From: Oliver Upton <oliver.upton@...ux.dev>
To: Shaoqin Huang <shahuang@...hat.com>
Cc: Marc Zyngier <maz@...nel.org>, kvmarm@...ts.linux.dev,
	Eric Auger <eauger@...hat.com>, Sebastian Ott <sebott@...hat.com>,
	Cornelia Huck <cohuck@...hat.com>,
	Catalin Marinas <catalin.marinas@....com>,
	James Morse <james.morse@....com>, kvm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org,
	Paolo Bonzini <pbonzini@...hat.com>, Shuah Khan <shuah@...nel.org>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Will Deacon <will@...nel.org>, Zenghui Yu <yuzenghui@...wei.com>
Subject: Re: [RFC PATCH v1 0/2] KVM: arm64: Making BT Field in
 ID_AA64PFR1_EL1 writable

Hi Shaoqin,

On Tue, Jun 11, 2024 at 10:35:50PM -0400, Shaoqin Huang wrote:
> Hi guys,
> 
> I'm trying to enable migration from MtCollins(Ampere Altra, ARMv8.2+) to
> AmpereOne(AmpereOne, ARMv8.6+), the migration always fails when migration from
> MtCollins to AmpereOne due to some register fields differing between the
> two machines.
> 
> In this patch series, we try to make more register fields writable like
> ID_AA64PFR1_EL1.BT. This is first step towards making the migration possible.
> Some other hurdles need to be overcome. This is not sufficient to make the
> migration successful from MtCollins to AmpereOne.

It isn't possible to transparently migrate between these systems. The
former has a cntfrq of 25MHz, and the latter has a cntfrq of 1GHz. There
isn't a mechanism for scaling the counter frequency, and I have zero
appetite for a paravirt interface.

On top of that, erratum AC03_CPU_38 seems to make a migration from
Neoverse-N1 to AmpereOne quite perilous, unless you hide FEAT_HAFDBS on
the source.

These issues are separate, though, from any possible changes to the
writability of ID_AA64PFR1_EL1, which still may be useful to userspace.

-- 
Thanks,
Oliver