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

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Message-ID: <50B8F4D7.2010204@zytor.com>
Date:	Fri, 30 Nov 2012 10:03:03 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Borislav Petkov <bp@...en8.de>, 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 11/30/2012 09:01 AM, Linus Torvalds wrote:
> On Thu, Nov 29, 2012 at 1:24 PM, H. Peter Anvin <hpa@...ux.intel.com> wrote:
>>
>> Thinking about it some more, there is another reason to not do this,
>> which is that we don't want this particular CPUID to be paravirtualized;
>> we're after the synchronizing side effect, not the CPUID return value
>> itself.
>>
>> So let's leave it as a primitive; it gets too confusing otherwise.
>
> Hmm. The virtualization issue brings up another point: do we *really*
> want to use cpuid for serialization at all?
>

Well, the grand total of serializing instructions are:

INVD, INVEPT, INVLPG, INVVPID, LGDT, LIDT, LLDT, LTR, MOV to CR, MOV to 
DR, WBINVD, WRMSR, CPUID, IRET, RSM.

It doesn't really leave a lot of wiggle room, and in the microcode case, 
the use of CPUID level 1 is actually mandated (presumably to get a 
uniform sequence for validation purposes.)

> 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.

This is still boot time, and I really doubt it is measurable in the long 
run.  Yes, exists suck, but at least CPUID is generally a quick exit, 
since all the relevant state is in registers.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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