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

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Date:	Fri, 30 Nov 2012 19:10:37 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...ux.intel.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 10:03:03AM -0800, H. Peter Anvin wrote:
> 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.

What about MFENCE with JMP 1f to kill prefetches? Probably too
expensive. Oh and MFENCE is an SSE2 insn so maybe not supported by 486
or something later ...

> >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.

Agreed.

-- 
Regards/Gruss,
    Boris.
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