Re: [PATCH 03/18] KVM: PPC: Teach MMIO Signedness

[Anthony Liguori <anthony@codemonkey.ws>]

On 02/07/2010 06:32 AM, Avi Kivity wrote:
> On 02/04/2010 05:55 PM, Alexander Graf wrote:
>> The guest I was trying to get to run uses the LHA and LHAU instructions.
>> Those instructions basically do a load, but also sign extend the result.
>>
>> Since we need to fill our registers by hand when doing MMIO, we also 
>> need
>> to sign extend manually.
>>
>> This patch implements sign extended MMIO and the LHA(U) instructions.
>>
>> @@ -300,6 +300,25 @@ static void kvmppc_complete_mmio_load(struct 
>> kvm_vcpu *vcpu,
>>           }
>>       }
>>
>> +    if (vcpu->arch.mmio_sign_extend) {
>> +        switch (run->mmio.len) {
>> +#ifdef CONFIG_PPC64
>> +        case 4:
>> +            if (gpr&  0x80000000)
>> +                gpr |= 0xffffffff00000000ULL;
>> +            break;
>
> Wouldn't
>
>   gpr = (s64)(gpr << 32) >> 32;
>
> work?  Not sure if >> is guaranteed to sign extend.

It's technically implementation dependent but I don't know of an 
implementation that doesn't sign extend.

Regards,

Anthony Liguori