Re: [Xen-devel] [PATCH 3/8] x86/time: introduce and use rdtsc_ordered()

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On Mon, 2016-06-20 at 13:59 +0100, Andrew Cooper wrote:
> On 15/06/16 11:27, Jan Beulich wrote:
> > Matching Linux commit 03b9730b76 ("x86/asm/tsc: Add rdtsc_ordered()
> > and
> > use it in trivial call sites") and earlier ones it builds upon,
> > let's
> > make sure timing loops don't have their rdtsc()-s re-ordered, as
> > that
> > would harm precision of the result (values were observed to be
> > several
> > hundred clocks off without this adjustment).
> > 
> > Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>  
> Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>
FWIW:

Reviewed-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
Tested-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>

(or Reviewed-and-Tested-by: as you wish :-)).

FTR, during my own investigation, before raising the issue on the
mailing list, I also came to the conclusion that we'd need something
like this. I even try doing something like this (in a much more hacky
way), and had the feeling that it was making a difference but, of
course, alone, without all the other issues that Jan found and fixed in
this series, it wasn't enough.

Thanks and regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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