> Case 3 is annoying. If nothing tries to change the user gs base, then > everything is okay because the user gs base and the kernel gs bases are > equal. But if something does try to change the user gs base, then it > will accidentally change the kernel gs base instead.
It doesn't really matter, as they are the same. They would just switch identities. Besides I don't think anyone does that. > > For the IST entries, this should be fine -- cpu migration, scheduling, > and such are impossible anyway. For the non-IST entries, I'm less > convinced. The entry_64.S code suggests that the problematic entries are: > > double_fault > stack_segment > machine_check I don't think any of them can schedule. > > Of course, all of those entries really do use IST, so I wonder why they > are paranoid*entry instead of paranoid*entry_ist. Is it because they're > supposedly non-recursive? Yes, only the DEBUG stack is big enough to recurse. > > In any case, wouldn't this all be much simpler and less magical if the > paranoid entries just saved the old gsbase to the rbx and loaded the new > ones? The exits could do the inverse. This should be really fast: I had it originally in a similar scheme, but it was significantly more complicated, with changed exit path So I switched to this "only a single hook needed" variant, which mirrors the existing code closely. > I don't know the actual latencies, but I suspect that this would be > faster, too -- it removes some branches, and wrgsbase and rdgsbase > deserve to be faster than swapgs. It's probably no good for > non-rd/wrgsbase-capable cpus, though, since I suspect that three MSR > accesses are much worse than one MSR access and two swapgs calls. Probably doesn't matter much, it's MUCH faster than the old code in any case. -Andi -- a...@linux.intel.com -- Speaking for myself only. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
