linux-kernel - Re: [PATCH 2/3] x86: Sanity-check static_cpu

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Date:	Mon, 29 Apr 2013 08:38:45 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <bp@...en8.de>
CC:	X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH 2/3] x86: Sanity-check static_cpu_has usage

On 04/29/2013 07:04 AM, Borislav Petkov wrote:
> From: Borislav Petkov <bp@...e.de>
> 
> static_cpu_has may be used only after alternatives have run. Before that
> it always returns false if constant folding with __builtin_constant_p()
> doesn't happen. And you don't want that.
> 
> This patch is the result of me debugging an issue where I overzealously
> put static_cpu_has in code which executed before alternatives have run
> and had to spend some time with scratching head and cursing at the
> monitor.
> 
> So add a jump to a warning which screams loudly when we use this
> function too early. The alternatives patch that check away in
> conjunction with patching the rest of the kernel image.
> 
> This first JMP the compiler then issues should always be a two-byte JMP
> because its relative offset fits in a byte. This gets replaced by only a
> two-byte NOP => less instruction cache bloat.
> 

I still would prefer this to be an option.

	-hpa


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