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Date:   Thu, 3 Jan 2019 14:48:35 -0800
From:   Andi Kleen <ak@linux.intel.com>
To:     Nadav Amit <nadav.amit@gmail.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>, X86 ML <x86@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Borislav Petkov <bp@alien8.de>,
        David Woodhouse <dwmw@amazon.co.uk>
Subject: Re: [RFC v2 1/6] x86: introduce kernel restartable sequence
Message-ID: <20190103224835.GG6118@tassilo.jf.intel.com>
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 <87va35e61a.fsf@linux.intel.com>
 <263A441C-F062-491F-9E95-F00FA2092A99@gmail.com>
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> 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.

-Andi