Hi folks. TLM is a communication protocol/mechanism built on top of systemc. It supports a mechanism called DMI which stands for direct memory interface. The idea is that an entity sending a request into the system can ask if the target can give it a pointer it can use to directly access that memory in the future. The target, if it supports that sort of thing, returns a descriptor which describes a region of memory that can be accessed in that way. If that needs to be invalidated in the future, then there's another mechanism the target can use to communicate back to the sender telling it to throw away that descriptor.
The way this mechanism is implemented in TLM is a bit less than ideal since every request has a field that says whether the requester wants to know about DMI, and so the target has to perform an extra check on all the requests in case someone is asking when that's useful to communicate only a very small fraction of the time, perhaps only once during an entire simulation. Aside from that though, this mechanism has some nice properties. First, it avoids having to globally identify what a memory is or where it is for a particular simulation. A memory is just a thing on the other end of a request that may let you get at it directly if you ask nicely. Also, if there's something in the way that would get messed up if you skipped over it, say a cache, it can block those requests from getting through to targets. This could be useful for KVM for instance, when it's collecting regions to act as RAM for the virtual machine. I haven't fully figured out what a good way to avoid the check-every-time problem of the systemc mechanism, and ideally whatever I/we come up with will be compatible enough to be bridged effectively, but I'm thinking some sort of explicit additional call like getAddrRanges which would propogate through the hierarchy at specific points, either to a specific address or as a broadcast. I know some folks have looked at gem5's memory system protocol and systemc's TLM before, for instance either to try making gem5 use TLM natively, or for the systemc TLM bridges. What do you think about adding this sort of mechainsm to gem5? Are there any pitfalls to avoid, known issues to figure out, suggested avenues to explore, etc? Please let me know. This is likely something I'm going to want to pursue in the next few weeks. Gabe _______________________________________________ gem5-dev mailing list gem5-dev@gem5.org http://m5sim.org/mailman/listinfo/gem5-dev
