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

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Date:   Thu, 3 Jan 2019 14:52:43 -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>,
        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 2:48 PM, Andi Kleen <ak@...ux.intel.com> wrote:
> 
>> 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.)
> 
> Are you sure actually? 
> 
> The empty prefix could mean 8bit register accesses.
> 
>>> 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?
> 
> Instruction encoding = system call ABI
> Upstream = CPU vendors
> 
> Early in Linux's history, naive Linux distribution vendors patched in their own
> private system calls without waiting for upstream to define an ABI, which caused
> endless compatibility problems. These days this is very frowned upon.
> 
>>> 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.
> 
> Write the address of the instruction into the per cpu variable.

Thanks for the explanations. I don’t think it would work (e.g., IRQs). I can
avoid generalizing and just detect the "magic sequence” of the code, but let
me give it some more thought.