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Message-ID: <07255D2B-0243-4254-B62A-37050C44207E@vmware.com>
Date: Thu, 18 Oct 2018 03:12:07 +0000
From: Nadav Amit <namit@...are.com>
To: Andy Lutomirski <luto@...capital.net>
CC: Ingo Molnar <mingo@...hat.com>, Andy 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>,
David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [RFC PATCH 1/5] x86: introduce preemption disable prefix
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.
> 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).
I agree that for super-general case this might be inappropriate.
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