From: Daniel Borkmann <daniel@iogearbox.net>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Thomas Gleixner" <tglx@linutronix.de>,
"Linux List Kernel Mailing" <linux-kernel@vger.kernel.org>,
Netdev <netdev@vger.kernel.org>,
"David S . Miller" <davem@davemloft.net>,
"Björn Töpel" <bjorn.topel@intel.com>,
"Magnus Karlsson" <magnus.karlsson@intel.com>,
"Jesper Dangaard Brouer" <brouer@redhat.com>,
"Alexei Starovoitov" <ast@kernel.org>,
"Peter Zijlstra" <peterz@infradead.org>,
"David Woodhouse" <dwmw2@infradead.org>,
"Andy Lutomirski" <luto@kernel.org>,
"Borislav Petkov" <bp@alien8.de>
Subject: Re: [PATCH] x86, retpolines: raise limit for generating indirect calls from switch-case
Date: Thu, 21 Feb 2019 23:56:55 +0100 [thread overview]
Message-ID: <d58b52ae-c4d9-991a-b53b-4047abe2cb56@iogearbox.net> (raw)
In-Reply-To: <CAHk-=wgNfnC-oe7aJGgNWoTbssc0LPYK6FyR=6LO4_U_dsphbg@mail.gmail.com>
On 02/21/2019 11:27 PM, Linus Torvalds wrote:
> On Thu, Feb 21, 2019 at 2:20 PM Daniel Borkmann <daniel@iogearbox.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
next prev parent reply other threads:[~2019-02-21 22:57 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-21 22:19 [PATCH] x86, retpolines: raise limit for generating indirect calls from switch-case Daniel Borkmann
2019-02-21 22:27 ` Linus Torvalds
2019-02-21 22:56 ` Daniel Borkmann [this message]
2019-02-22 7:31 ` Jesper Dangaard Brouer
2019-02-26 18:20 ` Björn Töpel
2019-02-28 11:12 ` [tip:x86/build] x86, retpolines: Raise " tip-bot for Daniel Borkmann
2019-02-28 11:27 ` David Woodhouse
2019-02-28 12:53 ` H.J. Lu
2019-02-28 16:18 ` Daniel Borkmann
2019-02-28 16:25 ` H.J. Lu
2019-02-28 17:58 ` Daniel Borkmann
2019-02-28 18:09 ` H.J. Lu
2019-02-28 18:13 ` Daniel Borkmann
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