Re: [RFC PATCH] KVM: arm64: Override Microsoft Azure Cobalt 100 MIDR value with ARM Neoverse N2

From: Mark Rutland
Date: Fri Feb 09 2024 - 06:34:12 EST

On Thu, Feb 08, 2024 at 11:16:10AM -0800, Easwar Hariharan wrote:
> On 2/7/2024 1:49 AM, Mark Rutland wrote:
> > On Tue, Feb 06, 2024 at 07:58:16PM +0000, Easwar Hariharan wrote:
> > Further, if Azure Cobalt 100 is based on ARM Neoverse N2, you presumably suffer
> > from the same errata; can you comment on that at all? e.g. are there any
> > changes in this part that *might* lead to differences in errata and/or
> > workarounds? How do the MIDR_EL1.{Variant,Revision} values compare to that of
> > Neoverse N2?
> Yes, Azure Cobalt 100 suffers from the same errata as Neoverse N2. We had changes
> in the implementation, but according to our hardware folks, the Neoverse N2 errata
> we are affected by so far aren't affected by the changes made for Azure Cobalt 100.

Ok, so of the currently-known-and-mitigated errata, you'll be affected by:


.. and we'll need to extend the midr_range lists for those errata to cover
Azure Cobalt 100.

>From your patch, it looks like the Azure Cobalt 100 MIDR value (0x6D0FD490) is
the same as the Arm Neoverse-N2 r0p0 MIDR value (0x410FD490), except the
'Implementer' field is 0x6D ('m' in ASCII) rather than 0x41 ('A' in ASCII).

Are you happy to send a patch extending arch/arm64/include/asm/cputype.h with
the relevant ARM_CPU_IMP_* and CPU_PART_* definitions, and use those to extend
the midr_range lists for those errata?

As above, if you could make any comment on how the MIDR_EL1.{Variant,Revision}
fields map to that of Arm Neoverse-N2, it would be very helpful. It's not clear
to me whether those fields correspond directly (and so this part is based on
r0p0), or whether you have a different scheme for revision numbers. That'll
matter for correctly matching any future errata and/or future revisions of
Azure Cobalt 100.