Hi Brad
I am trying to complete the patch on functional accesses in Ruby. I came
across this problem while testing the patch for higher number of
processors. I am working with MESI CMP directory protocol right now. So I
will describe the problem in its context.
Assume we are trying to functionally write some thing to block A. It is in
state S_I in the L2 cache. When a block moves to state S_I from state SS,
then the cache block in the cache is deallocated. Therefore, when viewed
from the CacheMemory's perpespective, since the cache does not have block
A, therefore, the L2 cache is of no consequence for this access. But the
controller has a TBE for this block. And this TBE will have this block
with AccessPermission:Busy. Also, there are L1 caches in the system that
hold this block in S state.
Now, as per the current condition for write functional accesses,
if((num_busy == 0 && num_ro > 0) || num_rw == 1)
this access would go ahead as num_busy would evaluate to 0 and num_ro
would evaluate to some value greater than 0. But clearly we do not want
this access to be performed since that state S_I is a busy state and no
other cache holds the block in a read-write state.
It seems to me that the controller should supply the function for deciding
the access permissions, since it is possible that one the TBE holds the
block.
--
Nilay
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