[PATCH] x86: introduce "hot" and "cold" page copying functions

[Jan Beulich]

The present copy_page_sse2() is useful in case the destination page isn't
going to get touched again soon, or if we want to limit churn on the
caches. Just rename it, to fit the corresponding {clear,scrub}_page_*()
naming scheme.

For cases where latency is the most important aspect, or when it is
expected that sufficiently large parts of a destination page will get
accessed again soon after the copying, introduce a "hot" alternative.
Again use alternatives patching to select between a "legacy" and an ERMS
variant.

Don't switch any callers just yet - this will be the subject of subsequent
changes.

Signed-off-by: Jan Beulich <jbeul...@suse.com>
---
Of course both pages can be independently hot/cold, and hence more flavors
may be necessary here.

To avoid the NOP padding (also in clear_page_hot()) we could use a double
REP prefix in the replacement code (accounting for the REX one in the code
being replaced).

--- a/xen/arch/x86/copy_page.S
+++ b/xen/arch/x86/copy_page.S
@@ -13,7 +13,7 @@
 #define tmp3_reg %r10
 #define tmp4_reg %r11
 
-FUNC(copy_page_sse2)
+FUNC(copy_page_cold)
         mov     $PAGE_SIZE/(4*WORD_SIZE)-3, %ecx
 
         prefetchnta 2*4*WORD_SIZE(src_reg)
@@ -44,4 +44,19 @@ FUNC(copy_page_sse2)
 
         sfence
         RET
-END(copy_page_sse2)
+END(copy_page_cold)
+
+        .macro copy_page_movsb
+        mov     $PAGE_SIZE, %ecx
+        rep movsb
+        .endm
+
+        .macro copy_page_movsq
+        mov     $PAGE_SIZE/8, %ecx
+        rep movsq
+        .endm
+
+FUNC(copy_page_hot)
+        ALTERNATIVE copy_page_movsq, copy_page_movsb, X86_FEATURE_ERMS
+        RET
+END(copy_page_hot)
--- a/xen/arch/x86/include/asm/page.h
+++ b/xen/arch/x86/include/asm/page.h
@@ -221,10 +221,11 @@ typedef struct { u64 pfn; } pagetable_t;
 
 void clear_page_hot(void *pg);
 void clear_page_cold(void *pg);
-void copy_page_sse2(void *to, const void *from);
+void copy_page_hot(void *to, const void *from);
+void copy_page_cold(void *to, const void *from);
 
 #define clear_page(_p)      clear_page_cold(_p)
-#define copy_page(_t, _f)   copy_page_sse2(_t, _f)
+#define copy_page(_t, _f)   copy_page_cold(_t, _f)
 
 #ifdef CONFIG_DEBUG
 void scrub_page_hot(void *ptr);