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SubjectRe: [RFC PATCH 1/5] x86: introduce preemption disable prefix
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at 10:00 AM, Andy Lutomirski <luto@amacapital.net> wrote:

>
>
>> On Oct 18, 2018, at 9:47 AM, Nadav Amit <namit@vmware.com> wrote:
>>
>> at 8:51 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>>
>>>> On Wed, Oct 17, 2018 at 8:12 PM Nadav Amit <namit@vmware.com> wrote:
>>>> at 6:22 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>>>>
>>>>>> On Oct 17, 2018, at 5:54 PM, Nadav Amit <namit@vmware.com> wrote:
>>>>>>
>>>>>> It is sometimes beneficial to prevent preemption for very few
>>>>>> instructions, or prevent preemption for some instructions that precede
>>>>>> a branch (this latter case will be introduced in the next patches).
>>>>>>
>>>>>> To provide such functionality on x86-64, we use an empty REX-prefix
>>>>>> (opcode 0x40) as an indication that preemption is disabled for the
>>>>>> following instruction.
>>>>>
>>>>> Nifty!
>>>>>
>>>>> That being said, I think you have a few bugs. First, you can’t just ignore
>>>>> a rescheduling interrupt, as you introduce unbounded latency when this
>>>>> happens — you’re effectively emulating preempt_enable_no_resched(), which
>>>>> is not a drop-in replacement for preempt_enable(). To fix this, you may
>>>>> need to jump to a slow-path trampoline that calls schedule() at the end or
>>>>> consider rewinding one instruction instead. Or use TF, which is only a
>>>>> little bit terrifying…
>>>>
>>>> Yes, I didn’t pay enough attention here. For my use-case, I think that the
>>>> easiest solution would be to make synchronize_sched() ignore preemptions
>>>> that happen while the prefix is detected. It would slightly change the
>>>> meaning of the prefix.
>>
>> So thinking about it further, rewinding the instruction seems the easiest
>> and most robust solution. I’ll do it.
>>
>>>>> You also aren’t accounting for the case where you get an exception that
>>>>> is, in turn, preempted.
>>>>
>>>> Hmm.. Can you give me an example for such an exception in my use-case? I
>>>> cannot think of an exception that might be preempted (assuming #BP, #MC
>>>> cannot be preempted).
>>>
>>> Look for cond_local_irq_enable().
>>
>> I looked at it. Yet, I still don’t see how exceptions might happen in my
>> use-case, but having said that - this can be fixed too.
>
> I’m not totally certain there’s a case that matters. But it’s worth checking
>
>> To be frank, I paid relatively little attention to this subject. Any
>> feedback about the other parts and especially on the high-level approach? Is
>> modifying the retpolines in the proposed manner (assembly macros)
>> acceptable?
>
> It’s certainly a neat idea, and it could be a real speedup.

Great. So I’ll try to shape things up, and I still wait for other comments
(from others).

I’ll just mention two more patches I need to cleanup (I know I still owe you some
work, so obviously it will be done later):

1. Seccomp trampolines. On my Ubuntu, when I run Redis, systemd installs 17
BPF filters on the Redis server process that are invoked on each
system-call. Invoking each one requires an indirect branch. The patch keeps
a per-process kernel code-page that holds trampolines for these functions.

2. Binary-search for system-calls. Use the per-process kernel code-page also
to hold multiple trampolines for the 16 common system calls of a certain
process. The patch uses an indirection table and a binary-search to find the
proper trampoline.

Thanks again,
Nadav
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