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Message-ID: <33bf951448e7d916fd4a6ad41cd3d040e9d1f118.camel@infradead.org>
Subject: Re: [tip:x86/build] x86, retpolines: Raise limit for generating
 indirect calls from switch-case
From:   David Woodhouse <dwmw2@infradead.org>
To:     daniel@iogearbox.net, mingo@kernel.org, bjorn.topel@intel.com,
        davem@davemloft.net, brouer@redhat.com, magnus.karlsson@intel.com,
        luto@kernel.org, hpa@zytor.com, tglx@linutronix.de,
        peterz@infradead.org, bp@alien8.de, torvalds@linux-foundation.org,
        linux-kernel@vger.kernel.org, ast@kernel.org,
        linux-tip-commits@vger.kernel.org
Cc:     hjl.tools@gmail.com
Date:   Thu, 28 Feb 2019 11:27:17 +0000
In-Reply-To: <tip-ce02ef06fcf7a399a6276adb83f37373d10cbbe1@git.kernel.org>
References: <20190221221941.29358-1-daniel@iogearbox.net>
         <tip-ce02ef06fcf7a399a6276adb83f37373d10cbbe1@git.kernel.org>
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On Thu, 2019-02-28 at 03:12 -0800, tip-bot for Daniel Borkmann wrote:
> Commit-ID:  ce02ef06fcf7a399a6276adb83f37373d10cbbe1
> Gitweb:     https://git.kernel.org/tip/ce02ef06fcf7a399a6276adb83f37373d1=
0cbbe1
> 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
>=20
> x86, retpolines: Raise limit for generating indirect calls from switch-ca=
se
>=20
> 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:
>=20
>   * 283c16a2dfd3 ("indirect call wrappers: helpers to speed-up indirect c=
alls 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")
>   [...]
>=20
> Recent work on XDP from Bj=C3=B6rn and Magnus additionally found that man=
ually
> 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.=20

+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=3D86952 so
please at least reference that bug, and be prepared to turn this hack
off when GCC is fixed.

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