From mboxrd@z Thu Jan  1 00:00:00 1970
From: Anthony Liguori <anthony-rdkfGonbjUSkNkDKm+mE6A@public.gmane.org>
Subject: Re: [PATCH 03/18] KVM: PPC: Teach MMIO Signedness
Date: Sun, 07 Feb 2010 10:27:03 -0600
Message-ID: <4B6EE9D7.5030608@codemonkey.ws>
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Cc: Alexander Graf <agraf-l3A5Bk7waGM@public.gmane.org>, kvm-ppc-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	kvm-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
To: Avi Kivity <avi-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
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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