Re: [RFC PATCH v1 0/2] KVM: arm64: Making BT Field in ID_AA64PFR1_EL1 writable

[Oliver Upton <oliver.upton@linux.dev>]

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