On Sat, Feb 11, 2023 at 9:59 PM Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: [...] > > is kind of why I want to understand the CAT, because for me > > explanation.txt is too much at a higher level to get a proper > > understanding of the memory model.. I tried re-reading explanation.txt > > many times.. then I realized I am just rewriting my own condensed set > > of notes every few months. > > Would you like to post a few examples showing some of the most difficult > points you encountered? Maybe explanation.txt can be improved. Just to list 2 of the pain points: 1. I think it is hard to reason this section "PROPAGATION ORDER RELATION: cumul-fence" All store-related fences should affect propagation order, even the smp_wmb() which is not A-cumulative should do so (po-earlier stores appearing before po-later). I think expanding this section with some examples would make sense to understand what makes "cumul-fence" different from any other store-related fence. 2. This part is confusing and has always confused me " The happens-before relation (hb) links memory accesses that have to execute in a certain order" It is not memory accesses that execute, it is instructions that execute. Can we separate out "memory access" from "instruction execution" in this description? I think ->hb tries to say that A ->hb B means, memory access A happened before memory access B exactly in its associated instruction's execution order (time order), but to be specific -- should that be instruction issue order, or instruction retiring order? AFAICS ->hb maps instruction execution order to memory access order. Not all ->po does fall into that category because of out-of-order hardware execution. As does not ->co because the memory subsystem may have writes to the same variable to be resolved out of order. It would be nice to call out that ->po is instruction issue order, which is different from execution/retiring and that's why it cannot be ->hb. ->rf does because of data flow causality, ->ppo does because of program structure, so that makes sense to be ->hb. IMHO, ->rfi should as well, because it is embodying a flow of data, so that is a bit confusing. It would be great to clarify more perhaps with an example about why ->rfi cannot be ->hb, in the "happens-before" section. That's really how far I typically get (line 1368) before life takes over, and I have to go do other survival-related things. Then I restart the activity. Now that I started reading the CAT file as well, I feel I can make it past that line :D. But I never wanted to get past it, till I built a solid understanding of the contents before it. As I read the file more, I can give more feedback, but the above are different 2 that persist. Thanks! - Joel
