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Date: Wed, 28 Sep 2011 10:24:33 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Linus Torvalds <torvalds@linux-foundation.org>
CC: Jeremy Fitzhardinge <jeremy@goop.org>, Jan Beulich <JBeulich@suse.com>,
        Stephan Diestelhorst <stephan.diestelhorst@amd.com>,
        Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>,
        Ingo Molnar <mingo@elte.hu>, Andi Kleen <andi@firstfloor.org>,
        Peter Zijlstra <peterz@infradead.org>, Nick Piggin <npiggin@kernel.dk>,
        the arch/x86 maintainers <x86@kernel.org>,
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Subject: Re: [Xen-devel] [PATCH 00/10] [PATCH RFC V2] Paravirtualized ticketlocks
References: <cover.1315878463.git.jeremy.fitzhardinge@citrix.com> <3300108.XQUp9Wrktc@chlor> <4E81FD52.50106@goop.org> <CA+55aFx7dv4C4ZB_3CFjdJmX_wpoxecpqo1ARYr1=zTVP=CVVA@mail.gmail.com> <4E835F8C0200007800058461@nat28.tlf.novell.com> <CA+55aFyVoZpZZp7ejypTv21Cx_qJWkZQdpJOnxBa_jUxsCjhuw@mail.gmail.com> <4E834FBA.1080709@goop.org> <CA+55aFzGZp8YaQMMUMwV+pd_vnqR14CtiTmTtkXuotQBg0zspg@mail.gmail.com>
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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@goop.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.