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

[H. Peter Anvin]

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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