Re: [Xen-devel] [PATCH v2] x86/mm: also flush TLB when putting writable foreign page reference

[Tim Deegan <tim@xxxxxxx>]

At 04:52 -0600 on 28 Apr (1493355160), Jan Beulich wrote:
> >>> On 27.04.17 at 11:51, <tim@xxxxxxx> wrote:
> > At 03:23 -0600 on 27 Apr (1493263380), Jan Beulich wrote:
> >> ... it wouldn't better be the other way around: We use the patch
> >> in its current (or even v1) form, and try to do something about
> >> performance only if we really find a case where it matters. To be
> >> honest, I'm not even sure how I could meaningfully measure the
> >> impact here: Simply counting how many extra flushes there would
> >> end up being wouldn't seem all that useful, and whether there
> >> would be any measurable difference in the overall execution time
> >> of e.g. domain creation I would highly doubt (but if it's that what
> >> you're after, I could certainly collect a few numbers).
> > 
> > I think that would be a good idea, just as a sanity-check.
> 
> As it turns out there is a measurable effect: xc_dom_boot_image()
> for a 4Gb PV guest takes about 70% longer now. Otoh it is itself
> responsible for less than 10% of the overall time libxl__build_dom()
> takes, and that in turn is only a pretty small portion of the overall
> "xl create".

Do you think that slowdown is OK?  I'm not sure -- I'd be inclined to
avoid it, but could be persuaded, and it's not me doing the work. :)
Andrew, what do you think?

Tim.

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