x86, boot: use appropriate rep string for move and clear

In the pre-decompression code, use the appropriate largest possible
rep movs and rep stos to move code and clear bss, respectively.  For
reverse copy, do note that the initial values are supposed to be the
address of the first (highest) copy datum, not one byte beyond the end
of the buffer.

rep strings are not necessarily the fastest way to perform these
operations on all current processors, but are likely to be in the
future, and perhaps more importantly, we want to encourage the
architecturally right thing to do here.

This also fixes a couple of trivial inefficiencies on 64 bits.

[ Impact: trivial performance enhancement, increase code similarity ]

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

2 files changed, 19 insertions, 18 deletions

diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
index d02a4f02be13..6710dc78ac59 100644
--- a/arch/x86/boot/compressed/head_32.S
+++ b/arch/x86/boot/compressed/head_32.S
@@ -100,11 +100,12 @@ ENTRY(startup_32)
  * where decompression in place becomes safe.
  */
         pushl   %esi
-        leal    _bss(%ebp), %esi
-        leal    _bss(%ebx), %edi
+        leal    (_bss-4)(%ebp), %esi
+        leal    (_bss-4)(%ebx), %edi
         movl    $(_bss - startup_32), %ecx
+        shrl    $2, %ecx
         std
-        rep     movsb
+        rep     movsl
         cld
         popl    %esi
 
@@ -135,8 +136,8 @@ relocated:
         leal    _bss(%ebx), %edi
         leal    _ebss(%ebx), %ecx
         subl    %edi, %ecx
-        cld
-        rep     stosb
+        shrl    $2, %ecx
+        rep     stosl
 
 /*
  * Do the decompression, and jump to the new kernel..
diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index a0b18426069a..723c72dfd7bc 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -260,15 +260,15 @@ ENTRY(startup_64)
  * Copy the compressed kernel to the end of our buffer
  * where decompression in place becomes safe.
  */
-        leaq    _bss(%rip), %r8
-        leaq    _bss(%rbx), %r9
+        pushq   %rsi
+        leaq    (_bss-8)(%rip), %rsi
+        leaq    (_bss-8)(%rbx), %rdi
         movq    $_bss /* - $startup_32 */, %rcx
-1:      subq    $8, %r8
-        subq    $8, %r9
-        movq    0(%r8), %rax
-        movq    %rax, 0(%r9)
-        subq    $8, %rcx
-        jnz     1b
+        shrq    $3, %rcx
+        std
+        rep     movsq
+        cld
+        popq    %rsi
 
 /*
  * Jump to the relocated address.
@@ -282,12 +282,12 @@ relocated:
 /*
  * Clear BSS (stack is currently empty)
  */
-        xorq    %rax, %rax
-        leaq    _bss(%rbx), %rdi
-        leaq    _ebss(%rbx), %rcx
+        xorl    %eax, %eax
+        leaq    _bss(%rip), %rdi
+        leaq    _ebss(%rip), %rcx
         subq    %rdi, %rcx
-        cld
-        rep     stosb
+        shrq    $3, %rcx
+        rep     stosq
 
 /*
  * Do the decompression, and jump to the new kernel..