> On Oct. 12, 2015, 9:58 a.m., Nilay Vaish wrote: > > Can you explain why it is no longer necessary? > > Curtis Dunham wrote: > Empirically, all regressions still pass. :-) > > It might be easier to explain why it was ever necessary, which is > definitely before my time with gem5. This check was introduced in 2011 > (3893d9d) with the comment "make sure fetch doesn't go off into the weeds > during speculation". This check is unrealistic and now o3 doesn't have any > issues because it can handle whatever faults come from bad fetch requests, as > it should.
I agree that this check is unrealistic, but "all regressions still pass" is (unfortunately) pretty far from a validation that the check is unnecessary. When you're misspeculating in kernel mode, and there's a TLB-bypass portion of the address space, you can end up fetching from just about anywhere (invalid physical addresses, I/O space, etc.). It would be ideal if we could tolerate all of these things properly in the memory system, but I'd feel a lot better if we knew exactly what circumstances prompted Ali to put this in there in the first place, and knew for a fact that that specific issue was now handled properly otherwise---the software application of Chesterton's Fence (https://en.wikipedia.org/wiki/Wikipedia:Chesterton%27s_fence ). - Steve ----------------------------------------------------------- This is an automatically generated e-mail. To reply, visit: http://reviews.gem5.org/r/3150/#review7357 ----------------------------------------------------------- On Oct. 12, 2015, 9:13 a.m., Curtis Dunham wrote: > > ----------------------------------------------------------- > This is an automatically generated e-mail. To reply, visit: > http://reviews.gem5.org/r/3150/ > ----------------------------------------------------------- > > (Updated Oct. 12, 2015, 9:13 a.m.) > > > Review request for Default. > > > Repository: gem5 > > > Description > ------- > > This check is no longer necessary. > > > Diffs > ----- > > src/cpu/o3/fetch.hh d90aec9435bd720f2af2ef809680608b3b6955c5 > src/cpu/o3/fetch_impl.hh d90aec9435bd720f2af2ef809680608b3b6955c5 > > Diff: http://reviews.gem5.org/r/3150/diff/ > > > Testing > ------- > > > Thanks, > > Curtis Dunham > > _______________________________________________ gem5-dev mailing list [email protected] http://m5sim.org/mailman/listinfo/gem5-dev
