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Re: [Xen-devel] [PATCH] x86/clear_page: Update clear_page_sse2() after dropping 32bit Xen


  • To: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Mon, 17 Jun 2019 21:04:52 +0100
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  • Cc: Edwin Török <edvin.torok@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Delivery-date: Mon, 17 Jun 2019 20:05:11 +0000
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On 17/06/2019 20:49, Andrew Cooper wrote:
> This code was never updated when the 32bit build of Xen was dropped.
>
>  * Expand the now-redundant ptr_reg macro.
>  * The number of iterations in the loop can be halfed by using 64bit writes,
>    without consuming any extra execution resource in the pipeline.  Adjust all
>    numbers/offsets appropriately.
>  * Replace dec with sub to avoid a eflags stall, and position it to be
>    macro-fused with the related jmp.
>  * With no need to preserve eflags across the body of the loop, replace lea
>    with add which has 1/3'rd the latency on basically all 64bit hardware.
>
> A quick userspace perf test on my Haswell dev box indicates that the old
> version takes ~1385 cycles on average (ignoring outliers), and the new version
> takes ~1060 cyles, or about 77% of the time.

And just for giggles, a rep stosq loop on this hardware is ~180 cycles,
which is more than 5 times better than the result of this patch (16% of
the time).

In some copious free time, clear/copy page ought to be come runtime
dependent on FastString being enabled, but I don't have time to organise
this right now.

~Andrew

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