On Wed, Jun 17, 2015 at 11:48 AM, Ingo Molnar <mi...@kernel.org> wrote: > > * Andy Lutomirski <l...@kernel.org> wrote: > >> This is incomplete, but it's finally good enough that I think it's >> time to get other opinions on it. It is a complete rewrite of the >> slow path code that handles exits to user mode. > > Modulo the small comments I made about the debug checks interface plus naming > details the structure and intention of this series gives me warm fuzzy > feelings. > >> The exit-to-usermode code is copied in several places and is written in a >> nasty >> combination of asm and C. It's not at all clear what it's supposed to do, >> and >> the way it's structured makes it very hard to work with. For example, it's >> not >> even clear why syscall exit hooks are called only once per syscall right now. >> (It seems to be a side effect of the way that rdi and rdx are handled in the >> asm >> loop, and it seems reliable, but it's still pointlessly complicated.) The >> existing code also makes context tracking overly complicated and hard to >> understand. Finally, it's nearly impossible for anyone to change what >> happens >> on exit to usermode, since the existing code is so fragile. > > Amen. > >> I tried to clean it up incrementally, but I decided it was too hard. Instead, >> this series just replaces the code. It seems to work. > > Any known bugs beyond UML build breakage? > >> Context tracking in particular works very differently now. The low-level >> entry >> code checks that we're in CONTEXT_USER and switches to CONTEXT_KERNEL. The >> exit >> code does the reverse. There is no need to track what CONTEXT_XYZ state we >> came >> from, because we already know. Similarly, SCHEDULE_USER is gone, since we >> can >> reschedule if needed by simply calling schedule() from C code. >> >> The main things that are missing are that I haven't done the 32-bit parts >> (anyone want to help?) and therefore I haven't deleted the old C code. I >> also >> think this may break UML for trivial reasons. >> >> Because I haven't converted the 32-bit code yet, all of the now-unnecessary >> unnecessary calls to exception_enter are still present in traps.c. >> >> IRQ context tracking is still duplicated. We should probably clean it up by >> changing the core code to supply something like >> irq_enter_we_are_already_in_context_kernel. >> >> Thoughts? > > So assuming you fix the UML build I'm inclined to go for it, even in this > incomplete form, to increase testing coverage.
Andy, can you please share the build breakage you're facing? I'll happily help you fixing it. -- Thanks, //richard -- 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/
