Re: [RFC/INCOMPLETE 00/13] x86: Rewrite exit-to-userspace code

[Andy Lutomirski]

On Wed, Jun 17, 2015 at 3:13 AM, Richard Weinberger
<richard.weinberger@xxxxxxxxx> wrote:
> On Wed, Jun 17, 2015 at 11:48 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>>
>> * Andy Lutomirski <luto@xxxxxxxxxx> 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.
>

The do_signal declaration in arch/um/include/shared/kern_util.h
conflicts with the one I added to arch/x86/include/asm/signal.h.  The
latter shouldn't really be included in UML, I think, but I didn't see
how to fix it.  Just renaming one of the functions would resolve the
conflict.

--Andy
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