linux-kernel - Re: [PATCH 8/8] x86, cleanups: Simplify sync

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Message-ID: <20121130172804.GC23034@liondog.tnic>
Date:	Fri, 30 Nov 2012 18:28:04 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Mario Gzuk <mariogzuk@...hnikz.de>
Subject: Re: [PATCH 8/8] x86, cleanups: Simplify sync_core() in the case of
 no CPUID

On Fri, Nov 30, 2012 at 09:01:50AM -0800, Linus Torvalds wrote:
> Hmm. The virtualization issue brings up another point: do we *really*
> want to use cpuid for serialization at all?
> 
> Exactly because under _real_ virtualization (as opposed to para-virt),
> it can cause unnecessary exits in a virtualized environment, no?

Well, in that case the function performs as advertized although it is a
very expensive serialization for virt. And virt guys hate exits.

> So I'm wondering if there is any better synchronizing instruction..
> 
> I guess sync_core() isn't used *that* much, so maybe we don't care,
> but I thought I'd ask...

Well, we use it in alternatives, MCE, intel_early_init, Intel
ucode, ftrace, and SGI's GRU thing.

>From all of the above, the alternatives case is kinda relevant for virt
where we do text_poke_early in a loop for every alternative section
so this could pile up to a bunch of vmexits depending on the emulated
hardware. Might be worth a replacement if it is noticeable in guests.

Hmm...

-- 
Regards/Gruss,
    Boris.
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