From: "Jan Beulich" <JBeulich@suse.com>
To: "Andrew Cooper" <andrew.cooper3@citrix.com>
Cc: xen-devel <xen-devel@lists.xenproject.org>,
"Edwin Török" <edvin.torok@citrix.com>, WeiLiu <wl@xen.org>,
"Roger Pau Monne" <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH] x86/clear_page: Update clear_page_sse2() after dropping 32bit Xen
Date: Tue, 18 Jun 2019 04:33:47 -0600 [thread overview]
Message-ID: <5D08BE0B0200007800239323@prv1-mh.provo.novell.com> (raw)
In-Reply-To: <1560800999-11592-1-git-send-email-andrew.cooper3@citrix.com>
>>> On 17.06.19 at 21:49, <andrew.cooper3@citrix.com> 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.
>
> Reported-by: Edwin Török <edvin.torok@citrix.com>
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Assuming you want this to go in despite your REP STOSQ remark
later on,
Reviewed-by: Jan Beulich <jbeulich@suse.com>
with one remark:
> --- a/xen/arch/x86/clear_page.S
> +++ b/xen/arch/x86/clear_page.S
> @@ -2,18 +2,16 @@
>
> #include <asm/page.h>
>
> -#define ptr_reg %rdi
> -
> ENTRY(clear_page_sse2)
> - mov $PAGE_SIZE/16, %ecx
> + mov $PAGE_SIZE/32, %ecx
> xor %eax,%eax
>
> -0: dec %ecx
> - movnti %eax, (ptr_reg)
> - movnti %eax, 4(ptr_reg)
> - movnti %eax, 8(ptr_reg)
> - movnti %eax, 12(ptr_reg)
> - lea 16(ptr_reg), ptr_reg
> +0: movnti %rax, 0(%rdi)
Could I talk you into leaving out this 0? Rather old gas actually emits
an 8-bit displacement when it's spelled like this.
Jan
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next prev parent reply other threads:[~2019-06-18 10:34 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-17 19:49 [Xen-devel] [PATCH] x86/clear_page: Update clear_page_sse2() after dropping 32bit Xen Andrew Cooper
2019-06-17 20:04 ` Andrew Cooper
2019-06-18 10:33 ` Jan Beulich [this message]
2019-06-18 10:35 ` Andrew Cooper
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