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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 17.06.19 at 21:49, <andrew.cooper3@xxxxxxxxxx> 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@xxxxxxxxxx>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Assuming you want this to go in despite your REP STOSQ remark
later on,
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
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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