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SubjectRe: [PATCH] x86: tell the world boot_params is 16-byte aligned
On Fri, May 08, 2015 at 03:42:33PM +0200, Rasmus Villemoes wrote:
> It doesn't matter much, but this disassembly makes me cry a little bit:
>
> ffffffff81f21223 <copy_bootdata>:
> ffffffff81f21223: 55 push %rbp
> ffffffff81f21224: 48 c7 c0 40 c2 02 82 mov $0xffffffff8202c240,%rax
> ffffffff81f2122b: 48 89 fe mov %rdi,%rsi
> ffffffff81f2122e: a8 01 test $0x1,%al
>
> The reason is that boot_params is defined with
> __attribute__((aligned(16))) in boot/main.c, but other translation
> units only see the packed attribute on the definition of struct
> boot_params, so assume the worst. Making the de facto alignment public

Wouldn't it be better if we put both attributes together, i.e.:

diff --git a/arch/x86/include/uapi/asm/bootparam.h b/arch/x86/include/uapi/asm/bootparam.h
index ab456dc233b5..d26953d896d2 100644
--- a/arch/x86/include/uapi/asm/bootparam.h
+++ b/arch/x86/include/uapi/asm/bootparam.h
@@ -155,7 +155,7 @@ struct boot_params {
__u8 _pad8[48]; /* 0xcd0 */
struct edd_info eddbuf[EDDMAXNR]; /* 0xd00 */
__u8 _pad9[276]; /* 0xeec */
-} __attribute__((packed));
+} __attribute__((packed, aligned(16)));

enum {
X86_SUBARCH_PC = 0,
---
so that they're concentrated in one place?

gcc seems to swallow it, I haven't checked the bloat decrease though,
when one would declare boot_params this way.

Thanks.

--
Regards/Gruss,
Boris.

ECO tip #101: Trim your mails when you reply.
--


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