linux-kernel - Re: [PATCH] x86, retpolines: raise limit for generating indirect calls from switch-case

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Message-ID: <d58b52ae-c4d9-991a-b53b-4047abe2cb56@iogearbox.net>
Date:   Thu, 21 Feb 2019 23:56:55 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Björn Töpel <bjorn.topel@...el.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH] x86, retpolines: raise limit for generating indirect
 calls from switch-case

On 02/21/2019 11:27 PM, Linus Torvalds wrote:
> On Thu, Feb 21, 2019 at 2:20 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>>
>> In case of gcc, this setting is controlled by case-values-threshold
>> which has an architecture global default that selects 4 or 5 (
> 
> Ack. For retpoline, that's much too low.
> 
> Patch looks sane, although it would be good to verify just which
> versions of gcc this works for. All versions with retpoline?

The feature was first added in gcc 4.7 [0], under "General Optimizer Improvements":

  Support for a new parameter --param case-values-threshold=n was added to allow
  users to control the cutoff between doing switch statements as a series of if
  statements and using a jump table.

>From what I can tell, original author (H.J. Lu) provided backports up to gcc 4.8
and distros seem to have pulled it from his github branch [1] as upstream gcc does
not handle backports for stable versions that old.

Thanks,
Daniel

  [0] https://www.gnu.org/software/gcc/gcc-4.7/changes.html
  [1] https://bugs.launchpad.net/ubuntu/+source/gcc-4.8/+bug/1749261
      https://github.com/hjl-tools/gcc/tree/hjl/indirect/gcc-4_8-branch/master