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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 28.09.11 at 17:38, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, Sep 27, 2011 at 9:44 AM, Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:
>>
>> I guess it comes down to throwing myself on the efficiency of some kind
>> of fence instruction.  I guess an lfence would be sufficient; is that
>> any more efficient than a full mfence?  At least I can make it so that
>> its only present when pv ticket locks are actually in use, so it won't
>> affect the native case.
> 
> Please don't play with fences, just do the final "addb" as a locked 
> instruction.
> 
> In fact, don't even use an addb, this whole thing is disgusting:
> 
>   movzwl (%rdi),%esi (esi:=0x0400)
>   addb   $0x2,(%rdi) (LOCAL copy of lock is now: 0x0402)
>   movzwl (%rdi),%eax (local forwarding from previous store: eax := 0x0402)
> 
> just use "lock xaddw" there too.

I'm afraid that's not possible, as that might carry from the low 8 bits
into the upper 8 ones, which must be avoided.

Jan


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