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

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Date:	Tue, 19 Jan 2016 00:05:54 +0100
From:	Borislav Petkov <bp@...e.de>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	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>,
	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 11:45:18AM -0800, H. Peter Anvin wrote:
> I think the two-byte optimization is the real issue if there is one at
> all.

Well, we do optimize the jumps for a couple releases now, see
recompute_jump(). So we would always get the short JMP, if made possible
by how gcc lays out the code so that the target is not far away. If the
JMP to the dynamic branch doesn't get nopped out anyway, that is.

We have "debug-alternative" to dump exactly what happens during
patching.

> I don't care about the inittext, and unless I'm misremembering
> completely altinstructions also get ejected.

Ah yes, they do.

> So I don't personally object to killing off the unsafe variant.

Ok.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--