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

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Date:   Mon, 31 Dec 2018 21:12:10 +0000
From:   Nadav Amit <namit@...are.com>
To:     Andy Lutomirski <luto@...nel.org>
CC:     Ingo Molnar <mingo@...hat.com>,
        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>, 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 Dec 31, 2018, at 12:08 PM, Andy Lutomirski <luto@...nel.org> wrote:
> 
> 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?

I will. Sorry for not updating the commit-log. The GCC plugin, and various
requests (that I am not sure I fully agree with) really caused me to spit
some blood.

>> +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.

Indeed, the code should call optpoline_restart_rseq() (or some other name,
once I fix the naming). I will fix it.