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Date:   Thu, 18 Oct 2018 09:54:05 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Nadav Amit <namit@vmware.com>, Ingo Molnar <mingo@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org, Nadav Amit <nadav.amit@gmail.com>,
        x86@kernel.org, Borislav Petkov <bp@alien8.de>,
        David Woodhouse <dwmw@amazon.co.uk>
Subject: Re: [RFC PATCH 1/5] x86: introduce preemption disable prefix
Message-ID: <20181018075405.GS3121@hirez.programming.kicks-ass.net>
References: <20181018005420.82993-1-namit@vmware.com>
 <20181018005420.82993-2-namit@vmware.com>
 <D247AB7C-2890-49CE-B102-163E5B368FC2@amacapital.net>
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On Wed, Oct 17, 2018 at 06:22:48PM -0700, Andy Lutomirski 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...

At which point we're very close to in-kernel rseq.