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

>>> On 10.08.10 at 14:31, 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,

CR0.TS covers both FP and XMM state.

> then lazy save might be a good approach.  If not, then I agree

Lazy save, even in the kernel, is used mainly for avoiding the user
context restore, not for dealing with in-kernel accesses to that
register state. It certainly can be made work, but again I'm
uncertain it's worth it.

> 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.)


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