Re: [Xen-devel] Re: Even faster page copy for Xen?

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 10/08/2010 13:31, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

>> You can do so if you feel like saving/restoring all necessary XMM
>> state isn't going to eat up all of the performance win...
> 
> Again excuse my x86 ignorance, but on some architectures
> floating point registers can be saved/restored "lazily"
> because there is a privileged bit that disables their use
> (which can be trapped and used as a "floating-point dirty" bit).
> Is there anything equivalent for the XMM state?  If so,
> then lazy save might be a good approach.  If not, then I agree
> that the state save/restore overhead might eat up the performance
> win.  (However, if we were to later use Linux memory compaction
> and NUMA page migration, the performance tradeoff might change
> to positive.)

We do lazy FPU/SSE restore already. But in any case, it is questionable how
much faster you can make a non-temporal and/or non-local bulk memory copy:
it ought to be bottlenecked on FSB bandwidth.

 -- Keir



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