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

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Date:   Thu, 3 Jan 2019 14:29:36 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Andi Kleen <ak@...ux.intel.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>, 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 Jan 3, 2019, at 2:21 PM, Andi Kleen <ak@...ux.intel.com> wrote:
> 
> Nadav Amit <namit@...are.com> writes:
> 
> I see another poor man's attempt to reinvent TSX.
> 
>> 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.
> 
> You cannot just assume something like that. x86 is a constantly
> evolving architecture. The prefix might well have meaning at
> some point.
> 
> Before doing something like that you would need to ask the CPU
> vendors to reserve the sequence you're using for software use.

Ok… I’ll try to think about another solution. Just note that this is just
used as a hint to avoid unnecessary lookups. (IOW, nothing will break if the
prefix is used.)

> You're doing the equivalent of patching a private system call
> into your own kernel without working with upstream, don't do that.

I don’t understand this comment though. Can you please explain?

> Better to find some other solution to do the restart.
> How about simply using a per cpu variable? That should be cheaper
> anyways.

The problem is that the per-cpu variable needs to be updated after the call
is executed, when we are already not in the context of the “injected” code.
I can increase it before the call, and decrease it after return - but this
can create (in theory) long periods in which the code is “unpatchable”,
increase the code size and slow performance.

Anyhow, I’ll give more thought. Ideas are welcomed.