linux-kernel - Re: [Xen-devel] [PATCH 00/10] [PATCH RFC V2] Paravirtualized ticketlocks

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Message-ID: <4E835851.7070502@zytor.com>
Date:	Wed, 28 Sep 2011 10:24:33 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Jan Beulich <JBeulich@...e.com>,
	Stephan Diestelhorst <stephan.diestelhorst@....com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Nick Piggin <npiggin@...nel.dk>,
	the arch/x86 maintainers <x86@...nel.org>,
	xen-devel@...ts.xensource.com, Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	KVM <kvm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 00/10] [PATCH RFC V2] Paravirtualized ticketlocks

On 09/28/2011 10:22 AM, Linus Torvalds wrote:
> On Wed, Sep 28, 2011 at 9:47 AM, Jeremy Fitzhardinge <jeremy@...p.org> wrote:
>>
>> Could do something like:
>>
>>        if (ticket->head >= 254)
>>                prev = xadd(&ticket->head_tail, 0xff02);
>>        else
>>                prev = xadd(&ticket->head_tail, 0x0002);
>>
>> to compensate for the overflow.
> 
> Oh wow. You havge an even more twisted mind than I do.
> 
> I guess that will work, exactly because we control "head" and thus can
> know about the overflow in the low byte. But boy is that ugly ;)
> 
> But at least you wouldn't need to do the loop with cmpxchg. So it's
> twisted and ugly, but migth be practical.
> 

I suspect it should be coded as -254 in order to use a short immediate
if that is even possible...

	-hpa

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

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