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

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Message-ID: <3D4E057B-AB03-4C12-B59D-774E8954C742@zytor.com>
Date:	Mon, 18 Jan 2016 15:13:00 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <bp@...e.de>
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 January 18, 2016 3:05:54 PM PST, Borislav Petkov <bp@...e.de> wrote:
>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.

The optimization has always been there, the question is how often it actually kicks in.
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