From: "H.J. Lu" <hjl.tools@gmail.com>
To: David Woodhouse <dwmw2@infradead.org>
Cc: daniel@iogearbox.net, Ingo Molnar <mingo@kernel.org>,
bjorn.topel@intel.com, David Miller <davem@davemloft.net>,
brouer@redhat.com, magnus.karlsson@intel.com,
Andy Lutomirski <luto@kernel.org>,
"H. Peter Anvin" <hpa@zytor.com>,
Thomas Gleixner <tglx@linutronix.de>,
Peter Zijlstra <peterz@infradead.org>,
Borislav Petkov <bp@alien8.de>,
Linus Torvalds <torvalds@linux-foundation.org>,
LKML <linux-kernel@vger.kernel.org>,
ast@kernel.org, linux-tip-commits@vger.kernel.org
Subject: Re: [tip:x86/build] x86, retpolines: Raise limit for generating indirect calls from switch-case
Date: Thu, 28 Feb 2019 04:53:34 -0800 [thread overview]
Message-ID: <CAMe9rOrcOVRWuNWy3xp7NMD=2UbGH+C5zifDhd=-AxrrzEVrmw@mail.gmail.com> (raw)
In-Reply-To: <33bf951448e7d916fd4a6ad41cd3d040e9d1f118.camel@infradead.org>
On Thu, Feb 28, 2019 at 3:27 AM David Woodhouse <dwmw2@infradead.org> wrote:
>
> On Thu, 2019-02-28 at 03:12 -0800, tip-bot for Daniel Borkmann wrote:
> > Commit-ID: ce02ef06fcf7a399a6276adb83f37373d10cbbe1
> > Gitweb: https://git.kernel.org/tip/ce02ef06fcf7a399a6276adb83f37373d10cbbe1
> > Author: Daniel Borkmann <daniel@iogearbox.net>
> > AuthorDate: Thu, 21 Feb 2019 23:19:41 +0100
> > Committer: Thomas Gleixner <tglx@linutronix.de>
> > CommitDate: Thu, 28 Feb 2019 12:10:31 +0100
> >
> > x86, retpolines: Raise limit for generating indirect calls from switch-case
> >
> > From networking side, there are numerous attempts to get rid of indirect
> > calls in fast-path wherever feasible in order to avoid the cost of
> > retpolines, for example, just to name a few:
> >
> > * 283c16a2dfd3 ("indirect call wrappers: helpers to speed-up indirect calls of builtin")
> > * aaa5d90b395a ("net: use indirect call wrappers at GRO network layer")
> > * 028e0a476684 ("net: use indirect call wrappers at GRO transport layer")
> > * 356da6d0cde3 ("dma-mapping: bypass indirect calls for dma-direct")
> > * 09772d92cd5a ("bpf: avoid retpoline for lookup/update/delete calls on maps")
> > * 10870dd89e95 ("netfilter: nf_tables: add direct calls for all builtin expressions")
> > [...]
> >
> > Recent work on XDP from Björn and Magnus additionally found that manually
> > transforming the XDP return code switch statement with more than 5 cases
> > into if-else combination would result in a considerable speedup in XDP
> > layer due to avoidance of indirect calls in CONFIG_RETPOLINE enabled
> > builds.
>
> +HJL
>
> This is a GCC bug, surely? It should know how expensive each
> instruction is, and choose which to use accordingly. That should be
> true even when the indirect branch "instruction" is a retpoline, and
> thus enormously expensive.
>
> I believe this is https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86952 so
> please at least reference that bug, and be prepared to turn this hack
> off when GCC is fixed.
We couldn't find a testcase to show jump table with indirect branch
is slower than direct branches.
--
H.J.
next prev parent reply other threads:[~2019-02-28 12:54 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
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 [this message]
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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