On 03/30/2016 10:08 AM, Sergey Fedorov wrote: > As of catching tb_flush() in cpu_exec() there have been three approaches > proposed. > > The first approach is to get rid of 'tb_invalidated_flag' and use > 'tb_flush_count'. Capture 'tb_flush_count' inside 'tb_lock' critical > section of cpu_exec() and compare it on each execution loop iteration > before trying to do tb_add_jump(). This would be simple and clear but it > would cost an extra load from a shared variable 'tb_flush_count' each > time we go over the execution loop. > > The second approach is to make 'tb_invalidated_flag' per-CPU. This > would be conceptually similar to what we have, but would give us thread > safety. With this approach, we need to be careful to correctly clear and > set the flag. > > The third approach is to mark each individual TB as valid/invalid. This > is what Emilio has in his MTTCG series [2]. Following this approach, we > could have very clean code with no extra overhead on the hot path. > However, it would require to mark all TBs as invalid on tb_flush(). > Given that tb_flush() is rare, it shouldn't be a significant overhead. > Also, there could be several options how to mark TB valid/invalid: > a dedicated flag could be introduced or some invalid value of > pc/cs_base/flags could be used.
I'm not really fond of this third option. Yes, tb_flush is rare on some targets, but for those with software managed TLBs they're much more common. See e.g. mips and sparc. Even when tb_flush is rare, there can be 500k-1M TBs live when the flush does happen. I simply cannot imagine that individually touching 1M variables performs as well as setting one global variable, or taking a global lock. r~
