Re: [RFC PATCH 1/5] x86: introduce preemption disable prefix

[Andy Lutomirski]

> On Oct 18, 2018, at 9:47 AM, Nadav Amit <namit@xxxxxxxxxx> wrote:
> 
> at 8:51 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> 
>>> On Wed, Oct 17, 2018 at 8:12 PM Nadav Amit <namit@xxxxxxxxxx> wrote:
>>> at 6:22 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>> 
>>>>> On Oct 17, 2018, at 5:54 PM, Nadav Amit <namit@xxxxxxxxxx> 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 canât just ignore
>>>> a rescheduling interrupt, as you introduce unbounded latency when this
>>>> happens â youâre 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 terrifyingâ
>>> 
>>> Yes, I didnât 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. Iâll do it.
> 
>>>> You also arenât 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 donât see how exceptions might happen in my
> use-case, but having said that - this can be fixed too.

Iâm not totally certain thereâs a case that matters.  But itâs 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?
> 

Itâs certainly a neat idea, and it could be a real speedup.

> Thanks,
> Nadav