Re: [Xen-devel] [PATCH] svm: Do not intercept RDTCS(P) when TSC scaling is supported by hardware

[Boris Ostrovsky <boris.ostrovsky@xxxxxxx>]

On 04/25/2012 01:14 PM, Keir Fraser wrote:
> On 25/04/2012 17:04, "Wei Huang"<wei.huang2@xxxxxxx>  wrote:
>
> > > I certainly don't feel comfortable ACKing it.  I'd like
> > > to see some testing that demonstrates the patch either improves
> > > functionality or performance without breaking other things.
> > > But if nobody else shares my concern, I don't feel that
> > > I have the right to block it either.
> > We can provide some rdtsc performance numbers. Regarding functionality,
> > it is relatively hard to prove unless Dan has some more specific ideas
> > of testing it. I think the hardware rdtsc scaling is inline with
> > software-based emulated approach.
> I've put it in to xen-unstable. I'm not sure about putting into 4.1 for the
> forthcoming release from that branch.
As far as performance numbers are concerned, with this patch applied 
(i.e. native execution) RDTSC instruction executed in a loop takes about 
150ns per iteration. That's including a few instructions for loop control.
With full SW emulation (pre-patch) the same loop is executed in roughly 6us.

Obviously this is not a realistic scenario but it gives a feel of what 
the patch provides.
-boris


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