Subject: Re: [PATCH] KVM: PPC: e500: Emulate power management control SPR

From: Alexander Graf <[email-masked]>
Date: Thu, 03 Jul 2014 14:03:01 +0200

On 30.06.14 20:20, Scott Wood wrote:
> On Mon, 2014-06-30 at 15:55 +0300, Mihai Caraman wrote:
>> For FSL e6500 core the kernel uses power management SPR register (PWRMGTCR0)
>> to enable idle power down for cores and devices by setting up the idle count
>> period at boot time. With the host already controlling the power management
>> configuration the guest could simply benefit from it, so emulate guest request
>> as nop.
>>
>> Signed-off-by: Mihai Caraman <[email-masked]>
>> ---
>>   arch/powerpc/kvm/e500_emulate.c | 8 ++++++++
>>   1 file changed, 8 insertions(+)
>>
>> diff --git a/arch/powerpc/kvm/e500_emulate.c b/arch/powerpc/kvm/e500_emulate.c
>> index 002d517..98a22e5 100644
>> --- a/arch/powerpc/kvm/e500_emulate.c
>> +++ b/arch/powerpc/kvm/e500_emulate.c
>> @@ -250,6 +250,10 @@ int kvmppc_core_emulate_mtspr_e500(struct kvm_vcpu *vcpu, int sprn, ulong spr_va
>>   				spr_val);
>>   		break;
>>   
>> +	case SPRN_PWRMGTCR0:
>> +		/* Guest relies on host power management configurations */
>> +		break;
>> +
>>   	/* extra exceptions */
>>   	case SPRN_IVOR32:
>>   		vcpu->arch.ivor[BOOKE_IRQPRIO_SPE_UNAVAIL] = spr_val;
>> @@ -355,6 +359,10 @@ int kvmppc_core_emulate_mfspr_e500(struct kvm_vcpu *vcpu, int sprn, ulong *spr_v
>>   		*spr_val = 0;
>>   		break;
>>   
>> +	case SPRN_PWRMGTCR0:
>> +		*spr_val = 0;
>> +		break;
>> +
>>   	case SPRN_MMUCFG:
>>   		*spr_val = vcpu->arch.mmucfg;
>>   		break;
> When reading, is it better to return zero, or the current host value, or
> the value last written by the guest (even though it wasn't written to
> hardware)?

I think it makes sense to treat it as general storage. I don't think 
leaking the host value into the guest is useful. And while zero works, 
the spec does say that the value gets retained, so I think we should do 
the same.

Alex