linux-kernel - Re: [RFC v2 1/6] x86: introduce kernel restartable sequence

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Message-ID: <CALCETrXqdgqYgsKd0xR7RMvVvotu1u==+n3gYyDgTZvbvmJ5VQ@mail.gmail.com>
Date:   Mon, 31 Dec 2018 12:08:15 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Nadav Amit <namit@...are.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Edward Cree <ecree@...arflare.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Nadav Amit <nadav.amit@...il.com>, X86 ML <x86@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [RFC v2 1/6] x86: introduce kernel restartable sequence

On Sun, Dec 30, 2018 at 11:20 PM Nadav Amit <namit@...are.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.

> +}