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

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.


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