Hi Arteen, If you are not tied to the MESI_three_level protocol, then, as Gautam said, you might want to switch to a different protocol that already supports the shared state in the system level cache. CHI is the most advanced and configurable protocol in that respect but others might do as well like MOESI_CMP_Directory.
If you must stick to MESI_three_level, then you will have to modify at least the directory to add a shared state that you transition to when sending read only data as a result of a GETS and there is only sharers (or no sharer at all). You will also have to modify the L2 to forward clean data as shared instead of exclusive. Several other modifications will be required along the way to issue the correct snoops, respond appropriately, etc. I am probably still overlooking several subtleties that will show up along the way, though. Overall, you are trying to change a pretty fundamental design aspect of the MESI protocol which is always assuming dirty data in caches from directory perspective. This enables certain silent transitions in the caches (typically, exclusive to modified) that will not be allowed in case the directory is in the shared state. If you still want to give it an attempt, you should use the ruby_mem_test.py coherency test configuration script together with the ProtocolTrace debug flag and possibly the Ruby debug flag for lower level issues. Regards, Gabriel
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