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

[Mark Rutland]

On Tue, Feb 06, 2024 at 07:58:16PM +0000, Easwar Hariharan wrote:
> Several workload optimizations and errata depend on validating that the
> optimization or errata are applicable to the particular CPU by checking
> the MIDR_EL1 system register value. With the Microsoft implementer ID
> for Azure Cobalt 100, the value doesn't match and ~20-25% performance
> regression is seen in these workloads. Override the Azure Cobalt 100
> value and replace it with the default ARM Neoverse N2 value that Azure
> Cobalt 100 is based on.

NAK to rewriting the MIDR in the kernel; we do not lie to userspace about the
MIDR, and this is not a can of worms we're going to open.

If you desire some microarchitectural performance optimizations in particular
projects, please submit patches to those projects to understand your MIDR
value.

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?

Mark.

> Signed-off-by: Easwar Hariharan <eahariha at linux.microsoft.com>
> ---
>  arch/arm64/include/asm/cputype.h   | 3 ++-
>  arch/arm64/include/asm/el2_setup.h | 5 +++++
>  arch/arm64/kvm/sys_regs.c          | 9 ++++++++-
>  3 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/cputype.h b/arch/arm64/include/asm/cputype.h
> index 7c7493cb571f..0450c6c32377 100644
> --- a/arch/arm64/include/asm/cputype.h
> +++ b/arch/arm64/include/asm/cputype.h
> @@ -262,7 +262,8 @@ is_midr_in_range_list(u32 midr, struct midr_range const *ranges)
>   */
>  static inline u32 __attribute_const__ read_cpuid_id(void)
>  {
> -	return read_cpuid(MIDR_EL1);
> +	return (read_cpuid(MIDR_EL1) == 0x6D0FD490 ? 0x410FD490 :
> +			read_cpuid(MIDR_EL1));
>  }
>  
>  static inline u64 __attribute_const__ read_cpuid_mpidr(void)
> diff --git a/arch/arm64/include/asm/el2_setup.h b/arch/arm64/include/asm/el2_setup.h
> index b7afaa026842..502a14e54a31 100644
> --- a/arch/arm64/include/asm/el2_setup.h
> +++ b/arch/arm64/include/asm/el2_setup.h
> @@ -138,6 +138,11 @@
>  .macro __init_el2_nvhe_idregs
>  	mrs	x0, midr_el1
>  	mrs	x1, mpidr_el1
> +	ldr	x2, =0x6D0FD490
> +	cmp	x0, x2
> +	bne	.Loverride_cobalt100_\@
> +	ldr	x0, =0x410FD490
> +.Loverride_cobalt100_\@:
>  	msr	vpidr_el2, x0
>  	msr	vmpidr_el2, x1
>  .endm
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 30253bd19917..8ea9c7fdabdb 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -3574,7 +3574,14 @@ id_to_sys_reg_desc(struct kvm_vcpu *vcpu, u64 id,
>  		return ((struct sys_reg_desc *)r)->val;			\
>  	}
>  
> -FUNCTION_INVARIANT(midr_el1)
> +static u64 get_midr_el1(struct kvm_vcpu *v, const struct sys_reg_desc *r)
> +{
> +	((struct sys_reg_desc *)r)->val = read_sysreg(midr_el1);
> +	if (((struct sys_reg_desc *)r)->val == 0x6D0FD490)
> +		((struct sys_reg_desc *)r)->val == 0x410FD490;
> +	return ((struct sys_reg_desc *)r)->val;
> +}
> +
>  FUNCTION_INVARIANT(revidr_el1)
>  FUNCTION_INVARIANT(aidr_el1)
>  
> -- 
> 2.34.1
> 
>