[Xen-changelog] [xen stable-4.12] x86/clear_page: Update clear_page_sse2() after dropping 32bit Xen

[patchbot@xxxxxxx]

commit d0a7bbe8dec85ec41c2f401c43548011c49873a0
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Fri Jul 19 16:12:06 2019 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Jul 19 16:12:06 2019 +0200

    x86/clear_page: Update clear_page_sse2() after dropping 32bit Xen
    
    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>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: 260acc521db4c29df4aa9b7a67f42cf967871fd3
    master date: 2019-06-18 15:47:52 +0100
---
 xen/arch/x86/clear_page.S | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/xen/arch/x86/clear_page.S b/xen/arch/x86/clear_page.S
index 243a7679c8..d9d524c79e 100644
--- 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,   (%rdi)
+        movnti  %rax,  8(%rdi)
+        movnti  %rax, 16(%rdi)
+        movnti  %rax, 24(%rdi)
+        add     $32, %rdi
+        sub     $1, %ecx
         jnz     0b
 
         sfence
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.12

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