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

[Richard Weinberger]

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.

-- 
Thanks,
//richard
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