linux-kernel - Re: [PATCH RFC V5 00/11] Paravirtualized ticketlocks

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Message-ID: <4E9C58D6.9010103@zytor.com>
Date:	Mon, 17 Oct 2011 09:33:26 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...nel.dk>, Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	KVM <kvm@...r.kernel.org>, Andi Kleen <andi@...stfloor.org>,
	Xen Devel <xen-devel@...ts.xensource.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: Re: [PATCH RFC V5 00/11] Paravirtualized ticketlocks

On 10/13/2011 09:44 AM, Jeremy Fitzhardinge wrote:
> 
> Yeah, that's a good question.  There are three mechanisms with somewhat
> overlapping concerns:
> 
>   * alternative()
>   * pvops patching
>   * jump_labels
> 
> Alternative() is for low-level instruction substitution, and really only
> makes sense at the assembler level with one or two instructions.
> 
> pvops is basically a collection of ordinary _ops structures full of
> function pointers, but it has a layer of patching to help optimise it. 
> In the common case, this just replaces an indirect call with a direct
> one, but in some special cases it can inline code.  This is used for
> small, extremely performance-critical things like cli/sti, but it
> awkward to use in general because you have to specify the inlined code
> as a parameterless asm.
> 
> Jump_labels is basically an efficient way of doing conditionals
> predicated on rarely-changed booleans - so it's similar to pvops in that
> it is effectively a very ordinary C construct optimised by dynamic code
> patching.

Then there is static_cpu_has(), which is basically jump labels
implemented using the alternatives mechanism.

If nothing else it would be good to:

1. Make more general use of ops patching;
2. Merge mechanisms where practical.

	-hpa

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

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