Re: [patch] SMP alternatives

[Zachary Amsden <zach@vmware.com>]

H. Peter Anvin wrote:

> Zachary Amsden wrote:
>
>>
>> You need a way to type the lock semantics by memory region, and a 
>> working hardware solution can not perform as well as a careful 
>> software solution.  As was pointed out earlier, you can't use memory 
>> type attributes to infer lock semantics, you must assume them in the 
>> decoder or implement complex deadlock detection and recovery in silicon.
>>
>
> Sure you can.  You just have to be prepared to take a microop 
> exception if you speculate incorrectly.


I spoke silicon too heavy handedly.  The complexity of the issue 
disappears if you take an exception, but rewinding state prior to the 
exception and reissuing is going to be less efficient than getting it 
right the first time, which is something software can always guarantee.  
You need to add more hardware for prediction to get it right all the 
time, and it is not clear the cost of that hardware is justified when 
software can always do the right thing.

Zach