linux-kernel - Re: [PATCH] x86: static_cpu_has_safe: discard dynamic check after init

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Date:	Mon, 18 Jan 2016 11:52:34 -0500
From:	Brian Gerst <brgerst@...il.com>
To:	Borislav Petkov <bp@...e.de>
Cc:	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86: static_cpu_has_safe: discard dynamic check after init

On Sun, Jan 17, 2016 at 5:33 AM, Borislav Petkov <bp@...e.de> wrote:
> On Sat, Jan 16, 2016 at 02:58:21PM -0500, Brian Gerst wrote:
>> It saves space.  The only thing left in main text is one jump
>> instruction (5 bytes).
>
> How do I measure this?
>
> Because with my tailored config here, the only thing I'm seeing is a growth of
> text by 730 bytes:
>
> before:
>    text    data     bss     dec     hex filename
> 10926552        3598944 16642048        31167544        1db9438 vmlinux
>
> [    0.056552] Freeing SMP alternatives memory: 24K (ffffffff81dd7000 - ffffffff81ddd000)
> [    2.883728] Freeing unused kernel memory: 2904K (ffffffff81b01000 - ffffffff81dd7000)
>
>
> after:
>    text    data     bss     dec     hex filename
> 10927282        3598944 16642048        31168274        1db9712 vmlinux
>
> [    0.052559] Freeing SMP alternatives memory: 24K (ffffffff81dd7000 - ffffffff81ddd000)
> [    3.225318] Freeing unused kernel memory: 2904K (ffffffff81b01000 - ffffffff81dd7000)

It is due to page alignment padding.  It was not enough to lose a
whole page from .text in your case.

The size command includes any section that is marked executable in the
text count, including init text.  If you use readelf -S vmlinux.o
instead you will notice that .text is the same size or smaller, and
.static_cpu_has (which is freed after boot) is the difference.

--
Brian Gerst