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

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Message-Id: <2626124E-7344-42F3-AD07-0BB34D62A9EE@amacapital.net>
Date:   Thu, 18 Oct 2018 10:00:37 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Nadav Amit <namit@...are.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        "Woodhouse, David" <dwmw@...zon.co.uk>
Subject: Re: [RFC PATCH 1/5] x86: introduce preemption disable prefix



> On Oct 18, 2018, at 9:47 AM, Nadav Amit <namit@...are.com> wrote:
> 
> at 8:51 PM, Andy Lutomirski <luto@...capital.net> wrote:
> 
>>> On Wed, Oct 17, 2018 at 8:12 PM Nadav Amit <namit@...are.com> wrote:
>>> at 6:22 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>> 
>>>>> On Oct 17, 2018, at 5:54 PM, Nadav Amit <namit@...are.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.

> Thanks,
> Nadav