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

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Date: Fri, 9 Feb 2024 11:33:51 +0000
From: Mark Rutland <mark.rutland@....com>
To: Easwar Hariharan <eahariha@...ux.microsoft.com>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>,
	James Morse <james.morse@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>,
	Andre Przywara <andre.przywara@....com>,
	Rob Herring <robh@...nel.org>, Fuad Tabba <tabba@...gle.com>,
	Joey Gouly <joey.gouly@....com>,
	Kristina Martsenko <kristina.martsenko@....com>,
	"moderated list:ARM64 PORT (AARCH64 ARCHITECTURE)" <linux-arm-kernel@...ts.infradead.org>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:KERNEL VIRTUAL MACHINE FOR ARM64 (KVM/arm64)" <kvmarm@...ts.linux.dev>
Subject: Re: [RFC PATCH] KVM: arm64: Override Microsoft Azure Cobalt 100 MIDR
 value with ARM Neoverse N2

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:

	ARM64_ERRATUM_2139208
	ARM64_ERRATUM_2067961
	ARM64_ERRATUM_2253138

.. 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.

Mark.