netdev - Re: [net-next 01/12] i40e: replace switch-statement to speed-up retpoline-enabled builds

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Message-ID: <5e546c17ad929d97d6c4ca7d93b8f504da33dc31.camel@intel.com>
Date:   Thu, 02 May 2019 13:40:00 -0700
From:   Jeff Kirsher <jeffrey.t.kirsher@...el.com>
To:     Björn Töpel <bjorn.topel@...el.com>,
        Daniel Borkmann <daniel@...earbox.net>, davem@...emloft.net
Cc:     netdev@...r.kernel.org, nhorman@...hat.com, sassmann@...hat.com,
        Andrew Bowers <andrewx.bowers@...el.com>
Subject: Re: [net-next 01/12] i40e: replace switch-statement to speed-up
 retpoline-enabled builds

On Thu, 2019-05-02 at 22:29 +0200, Björn Töpel wrote:
> On 2019-05-02 16:47, Daniel Borkmann wrote:
> > On 04/29/2019 09:16 PM, Jeff Kirsher wrote:
> > > From: Björn Töpel <bjorn.topel@...el.com>
> > > 
> > > GCC will generate jump tables for switch-statements with more than 5
> > > case statements. An entry into the jump table is an indirect call,
> > > which means that for CONFIG_RETPOLINE builds, this is rather
> > > expensive.
> > > 
> > > This commit replaces the switch-statement that acts on the XDP
> > > program
> > > result with an if-clause.
> > > 
> > > The if-clause was also refactored into a common function that can be
> > > used by AF_XDP zero-copy and non-zero-copy code.
> > 
> > Isn't it fixed upstream by now already (also in gcc)?
> > 
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ce02ef06fcf7a399a6276adb83f37373d10cbbe1
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a9d57ef15cbe327fe54416dd194ee0ea66ae53a4
> > 
> 
> Hmm, given that Daniel's work is upstream, this patch doesn't really
> make sense any more. OTOH it can stay in the series, and be cleaned up
> later.
> 
> I'll leave it for you to decide, Jeff!

I am already making revisions to the series due to another patch, so if
these changes are no longer needed to improve performance in RETPOLINE
builds, then lets drop it.

Björn, can you confirm that with or without these changes, XDP performance
stays the same for RETPOLINE builds?

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