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From:   Andy Lutomirski <luto@kernel.org>
Date:   Mon, 31 Dec 2018 12:08:15 -0800
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Subject: Re: [RFC v2 1/6] x86: introduce kernel restartable sequence
To:     Nadav Amit <namit@vmware.com>
Cc:     Ingo Molnar <mingo@redhat.com>, Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Edward Cree <ecree@solarflare.com>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        Nadav Amit <nadav.amit@gmail.com>, X86 ML <x86@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Borislav Petkov <bp@alien8.de>,
        David Woodhouse <dwmw@amazon.co.uk>
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On Sun, Dec 30, 2018 at 11:20 PM Nadav Amit <namit@vmware.com> wrote:
>
> It is sometimes beneficial to have a restartable sequence - very few
> instructions which if they are preempted jump to a predefined point.
>
> To provide such functionality on x86-64, we use an empty REX-prefix
> (opcode 0x40) as an indication for instruction in such a sequence. Before
> calling the schedule IRQ routine, if the "magic" prefix is found, we
> call a routine to adjust the instruction pointer.  It is expected that
> this opcode is not in common use.
>
> The following patch will make use of this function. Since there are no
> other users (yet?), the patch does not bother to create a general
> infrastructure and API that others can use for such sequences. Yet, it
> should not be hard to make such extension later.

The following patch does not use it.  Can you update this?


> +asmlinkage __visible void restart_kernel_rseq(struct pt_regs *regs)
> +{
> +       if (user_mode(regs) || *(u8 *)regs->ip != KERNEL_RESTARTABLE_PREFIX)
> +               return;

else?

I suspect something is missing here.  Or I'm very confused.

> +}