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

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 18/06/2019 11:33, Jan Beulich wrote:
>>>> 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.

Oh ok.  I'll still align the (%rdi) though to make the column easier to
read.

I'll put this in now, and see if I can find some time before 4.13 ships
to make some alternatives-based better options.

~Andrew

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