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

[Nadav Amit <namit@vmware.com>]

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.

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?

Thanks,
Nadav