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Date: Tue, 11 Jun 2024 22:30:51 -0700
From: Oliver Upton <oliver.upton@linux.dev>
To: Shaoqin Huang <shahuang@redhat.com>
Cc: Marc Zyngier <maz@kernel.org>, kvmarm@lists.linux.dev,
	Eric Auger <eauger@redhat.com>, Sebastian Ott <sebott@redhat.com>,
	Cornelia Huck <cohuck@redhat.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	James Morse <james.morse@arm.com>, kvm@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
	linux-kselftest@vger.kernel.org,
	Paolo Bonzini <pbonzini@redhat.com>, Shuah Khan <shuah@kernel.org>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Will Deacon <will@kernel.org>, Zenghui Yu <yuzenghui@huawei.com>
Subject: Re: [RFC PATCH v1 0/2] KVM: arm64: Making BT Field in
 ID_AA64PFR1_EL1 writable
Message-ID: <Zmkyi39Pz6Wqll-7@linux.dev>
References: <20240612023553.127813-1-shahuang@redhat.com>
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In-Reply-To: <20240612023553.127813-1-shahuang@redhat.com>

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