[Xen-devel] [PATCH 6/6] x86/boot: Ensure the BSS is aligned on an 8 byte boundary

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

... and zero it more efficiently.

A diff of the resulting binaries shows that the size of the BSS doesn't
actually change from the extra ALIGN()s, but they do guarantee that it is safe
to clear in an more efficient manner than 1 byte at a time.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Keir Fraser <keir@xxxxxxx>
CC: Jan Beulich <JBeulich@xxxxxxxx>
---
 xen/arch/x86/boot/head.S |    3 ++-
 xen/arch/x86/xen.lds.S   |    2 ++
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index 2d0e56c..b5a1d45 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -127,7 +127,8 @@ __start:
         mov     $sym_phys(__bss_end),%ecx
         sub     %edi,%ecx
         xor     %eax,%eax
-        rep     stosb
+        shr     $2,%ecx
+        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 4699a04..b1926e3 100644
--- a/xen/arch/x86/xen.lds.S
+++ b/xen/arch/x86/xen.lds.S
@@ -163,6 +163,7 @@ SECTIONS
   __init_end = .;
 
   .bss : {                     /* BSS */
+       . = ALIGN(8);
        __bss_start = .;
        *(.bss.stack_aligned)
        . = ALIGN(PAGE_SIZE);
@@ -175,6 +176,7 @@ SECTIONS
        *(.bss.percpu.read_mostly)
        . = ALIGN(SMP_CACHE_BYTES);
        __per_cpu_data_end = .;
+       . = ALIGN(8);
        __bss_end = .;
   } :text
   _end = . ;
-- 
1.7.10.4


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