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

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Date:   Thu, 18 Oct 2018 18:14:09 +0000
From:   Nadav Amit <namit@...are.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>
CC:     Ingo Molnar <mingo@...hat.com>, Andy Lutomirski <luto@...nel.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>,
        David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [RFC PATCH 1/5] x86: introduce preemption disable prefix

at 12:54 AM, Peter Zijlstra <peterz@...radead.org> wrote:

> On Wed, Oct 17, 2018 at 06:22:48PM -0700, Andy Lutomirski 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...
> 
> At which point we're very close to in-kernel rseq.

Interesting. .I didn’t know about this feature. I’ll see if I can draw some
ideas from there.