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

[Easwar Hariharan]

On 2/6/2024 11:54 PM, Anshuman Khandual wrote:
> 
> On 2/7/24 01:28, 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
> 
> Which is how it should be done.
> 
>> 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.
> 
> Why cannot these MIDR values be classified as required and subscribed to
> the existing erratas that is affecting such implementations. Hence these
> work arounds will be triggered as and when applicable. Why then override
> MIDR value instead ?
>

Thanks for the feedback, I will go ahead and add the Azure Cobalt 100 MIDR
value to the range of MIDRs affected by the Neoverse N2 errata. This
patch was a proof of concept to have userspace apply the Neoverse N2
optimizations to Azure Cobalt 100 as well. As Mark mentioned in a sibling
response, this is not an acceptable way to accomplish this.

>>
>> 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)
>>