asm alternatives: remove incorrect alignment notes
Linus Torvalds [Thu, 15 Sep 2011 20:28:33 +0000 (13:28 -0700)]
On x86-64, they were just wasteful: with the explicitly added (now
unnecessary) padding, the size of the alternatives structure was 16
bytes, and an alignment of 8 bytes didn't hurt much.

However, it was still silly, since the natural size and alignment for
the structure is actually just 12 bytes, 4-byte aligned since commit
59e97e4d6fbc ("x86: Make alternative instruction pointers relative").
So removing the padding, and removing the extra alignment is just a good
idea.

On x86-32, the alignment of 4 bytes was correct, but was incorrectly
hardcoded as 8 bytes in <asm/alternative-asm.h>.  That header file had
used to be an x86-64 only header file, but various unification efforts
have made it be used for x86-32 too (ie the unification of rwlock and
rwsem).

That in turn caused x86-32 boot failures, because the extra alignment
would result in random zero-filled words in the altinstructions section,
causing oopses early at boot when doing alternative instruction
replacement.

So just remove all the alignment noise entirely.  It's wrong, and it's
unnecessary.  The section itself is already properly aligned by the
linker scripts, and all additions to the section had better be of the
proper 12-byte format, keeping it aligned.  So if the align directive
were to ever make a difference, that would be an indication of a serious
bug to begin with.

Reported-by: Werner Landgraf <w.landgraf@ru.r>
Acked-by: Andrew Lutomirski <luto@mit.edu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/include/asm/alternative-asm.h
arch/x86/include/asm/alternative.h
arch/x86/include/asm/cpufeature.h

index 4554cc6..091508b 100644 (file)
@@ -16,7 +16,6 @@
 #endif
 
 .macro altinstruction_entry orig alt feature orig_len alt_len
-       .align 8
        .long \orig - .
        .long \alt - .
        .word \feature
index 23fb6d7..37ad100 100644 (file)
@@ -48,9 +48,6 @@ struct alt_instr {
        u16 cpuid;              /* cpuid bit set for replacement */
        u8  instrlen;           /* length of original instruction */
        u8  replacementlen;     /* length of new instruction, <= instrlen */
-#ifdef CONFIG_X86_64
-       u32 pad2;
-#endif
 };
 
 extern void alternative_instructions(void);
@@ -83,7 +80,6 @@ static inline int alternatives_text_reserved(void *start, void *end)
                                                                        \
       "661:\n\t" oldinstr "\n662:\n"                                   \
       ".section .altinstructions,\"a\"\n"                              \
-      _ASM_ALIGN "\n"                                                  \
       "         .long 661b - .\n"                      /* label           */   \
       "         .long 663f - .\n"                      /* new instruction */   \
       "         .word " __stringify(feature) "\n"      /* feature bit     */   \
index 4258aac..88b23a4 100644 (file)
@@ -332,7 +332,6 @@ static __always_inline __pure bool __static_cpu_has(u16 bit)
                asm goto("1: jmp %l[t_no]\n"
                         "2:\n"
                         ".section .altinstructions,\"a\"\n"
-                        _ASM_ALIGN "\n"
                         " .long 1b - .\n"
                         " .long 0\n"           /* no replacement */
                         " .word %P0\n"         /* feature bit */
@@ -350,7 +349,6 @@ static __always_inline __pure bool __static_cpu_has(u16 bit)
                asm volatile("1: movb $0,%0\n"
                             "2:\n"
                             ".section .altinstructions,\"a\"\n"
-                            _ASM_ALIGN "\n"
                             " .long 1b - .\n"
                             " .long 3f - .\n"
                             " .word %P1\n"             /* feature bit */