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

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Message-ID: <CALCETrVABHeHGyhxaATpSGcJX60f2rvVtVFc-5YQKQip-04qoQ@mail.gmail.com>
Date:	Mon, 18 Jan 2016 10:29:24 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...e.de>
Cc:	Brian Gerst <brgerst@...il.com>,
	"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>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86: static_cpu_has_safe: discard dynamic check after init

On Mon, Jan 18, 2016 at 10:14 AM, Borislav Petkov <bp@...e.de> wrote:
> On Mon, Jan 18, 2016 at 11:52:34AM -0500, Brian Gerst wrote:
>> It is due to page alignment padding.  It was not enough to lose a
>> whole page from .text in your case.
>
> So nothing more got freed.
>
>> 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.
>
> So we're talking about less than a page here?
>
>   [19] .static_cpu_has   PROGBITS         ffffffff81d335d3  011335d3
>        00000000000002df  0000000000000000  AX       0     0     1
>
> That's 479 bytes. Meh, it doesn't look like it is worth the trouble.
>

I think that, if we can make static_cpu_has be unconditionally safe as
a result and get rid of warn_pre_alternatives, then it is worth the
trouble.

--Andy