[Xen-devel] [PATCH v3 02/16] x86: zero BSS using stosl instead of stosb

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

Speedup BSS initialization by using stosl instead of stosb.

Some may argue that Intel Ivy Bridge and later provide ERMSB feature.
This means that "rep stosb" gives better throughput than "rep stosl" on
above mentioned CPUs. However, this feature is only available on newer
Intel processors and e.g. AMD does not provide it at all. So, stosb will
just give real benefits and even beat stosl only on limited number of
machines. On the other hand stosl will speedup BSS initialization on
all x86 platforms. Hence, use stosl instead of stosb.

Additionally, align relevant comment to coding style.

Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Daniel Kiper <daniel.kiper@xxxxxxxxxx>
---
v3 - suggestions/fixes:
   - improve comments
     (suggested by Konrad Rzeszutek Wilk),
   - improve commit message
     (suggested by Jan Beulich).
---
 xen/arch/x86/boot/head.S |    5 +++--
 xen/arch/x86/xen.lds.S   |    3 +++
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index f3501fd..32a54a0 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -123,12 +123,13 @@ __start:
         call    reloc
         mov     %eax,sym_phys(multiboot_ptr)
 
-        /* Initialize BSS (no nasty surprises!) */
+        /* Initialize BSS (no nasty surprises!). */
         mov     $sym_phys(__bss_start),%edi
         mov     $sym_phys(__bss_end),%ecx
         sub     %edi,%ecx
+        shr     $2,%ecx
         xor     %eax,%eax
-        rep     stosb
+        rep     stosl
 
         /* Interrogate CPU extended features via CPUID. */
         mov     $0x80000000,%eax
diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
index 961f48f..6802da1 100644
--- a/xen/arch/x86/xen.lds.S
+++ b/xen/arch/x86/xen.lds.S
@@ -191,6 +191,8 @@ SECTIONS
        CONSTRUCTORS
   } :text
 
+  /* Align BSS to speedup its initialization. */
+  . = ALIGN(4);
   .bss : {                     /* BSS */
        . = ALIGN(STACK_SIZE);
        __bss_start = .;
@@ -205,6 +207,7 @@ SECTIONS
        *(.bss.percpu.read_mostly)
        . = ALIGN(SMP_CACHE_BYTES);
        __per_cpu_data_end = .;
+       . = ALIGN(4);
        __bss_end = .;
   } :text
   _end = . ;
-- 
1.7.10.4


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