Re: [PATCH 01/15] KVM: PPC: Make register read/write wrappers always work

[Avi Kivity <avi-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On 03/08/2010 03:44 PM, Alexander Graf wrote:
> Avi Kivity wrote:
>    
>> On 03/05/2010 06:50 PM, Alexander Graf wrote:
>>      
>>> We have wrappers to do for example gpr read/write accesses with,
>>> because the contents of registers could be either in the PACA
>>> or in the VCPU struct.
>>>
>>> There's nothing that says we have to have the guest vcpu loaded
>>> when using these wrappers though, so let's introduce a flag that
>>> tells us whether we're inside a vcpu_load context.
>>>
>>>
>>>        
>> On x86 we always access registers within vcpu_load() context.  That
>> simplifies things.  Does this not apply here?
>>
>> Even so, sometimes guest registers are present on the cpu, and
>> sometimes in shadow variables (for example, msrs might be loaded or
>> not).  The approach here is to always unload and access the variable
>> data.  See for example vmx_set_msr() calling vmx_load_host_state()
>> before accessing msrs.
>>
>> Seems like this could reduce the if () tree?
>>      
> Well - it would probably render this particular patch void. In fact, I
> think it is already useless thanks to the other "always do vcpu_load" patch.
>
> As far as the already existing if goes, we can't really get rid of that.
> I want to be fast in the instruction emulation. Copying around the
> registers won't help there.
>    

So do it the other way around.  Always load the registers (of course, do 
nothing if already loaded) and then access them in just one way.  I 
assume during emulation the registers will always be loaded?

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