linux-kernel - Re: [PATCH] x86: tell the world boot

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Message-ID: <20150508142453.GC4137@pd.tnic>
Date:	Fri, 8 May 2015 16:24:53 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [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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