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

To: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH 00/10] [PATCH RFC V2] Paravirtualized ticketlocks
From: "H. Peter Anvin" <hpa@xxxxxxxxx>
Date: Wed, 28 Sep 2011 10:24:33 -0700
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>, Nick Piggin <npiggin@xxxxxxxxx>, KVM <kvm@xxxxxxxxxxxxxxx>, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, the arch/x86 maintainers <x86@xxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Marcelo Tosatti <mtosatti@xxxxxxxxxx>, Andi Kleen <andi@xxxxxxxxxxxxxx>, Avi Kivity <avi@xxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Ingo Molnar <mingo@xxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, Stephan Diestelhorst <stephan.diestelhorst@xxxxxxx>
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On 09/28/2011 10:22 AM, Linus Torvalds wrote:
> On Wed, Sep 28, 2011 at 9:47 AM, Jeremy Fitzhardinge <jeremy@xxxxxxxx> 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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