[Qemu-devel] Re: [PATCH] qemu/pci: optimize pci config handling

[Michael S. Tsirkin]

On Thu, Oct 08, 2009 at 10:23:41AM +0200, Paolo Bonzini wrote:
>
>>>> What you porpose here is certainly more code than we had.
>>>> So why is this a good idea?
>>>
>>> Because it avoids the memcpy/memcmp most of the time (when the memcmp
>>> would surely succeed).
>>
>> Yes :) But at the cost of more code.  I don't think speed
>> matters there, so less code is good.
>
> Fine.
>
>>>   I supposed that would also matter more as the
>>> config space size increases---correct me and dismiss the patch if I am
>>> mistaken.
>>
>> No, we'll always only look need to look at the header, whatever the size
>> of the config space.  That's the point of the patch I posted - future
>> proof against config space size increases, not optimization.
>
> But fewer reads on average will not modify the header, so there will be  
> even fewer memcpy with my patch when the config space will be 4k.

Oh, I see. I was worrying about us adding some side effect where you
write into PCI extended register and header changes.  Maybe that won't
ever happen.

> Paolo