netdev - Re: [Intel-wired-lan] [PATCH net-next v2 03/12] iavf: optimize Rx buffer allocation a bunch

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Date: Tue, 6 Jun 2023 07:24:47 -0700
From: Alexander Duyck <alexander.duyck@...il.com>
To: Alexander Lobakin <aleksander.lobakin@...el.com>
Cc: Paul Menzel <pmenzel@...gen.mpg.de>, Jesper Dangaard Brouer <hawk@...nel.org>, 
	Larysa Zaremba <larysa.zaremba@...el.com>, netdev@...r.kernel.org, 
	Ilias Apalodimas <ilias.apalodimas@...aro.org>, linux-kernel@...r.kernel.org, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Michal Kubiak <michal.kubiak@...el.com>, intel-wired-lan@...ts.osuosl.org, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Christoph Hellwig <hch@....de>, 
	Magnus Karlsson <magnus.karlsson@...el.com>
Subject: Re: [Intel-wired-lan] [PATCH net-next v2 03/12] iavf: optimize Rx
 buffer allocation a bunch

On Tue, Jun 6, 2023 at 5:49 AM Alexander Lobakin
<aleksander.lobakin@...el.com> wrote:
>
> From: Alexander Duyck <alexander.duyck@...il.com>
> Date: Fri, 2 Jun 2023 10:50:02 -0700
>
> Sorry for the silence, had sorta long weekend :p
>
> > On Fri, Jun 2, 2023 at 9:16 AM Alexander Lobakin
> > <aleksander.lobakin@...el.com> wrote:
>

[...]

> > The other thing is you may want to double check CPU(s) you are
> > expected to support as last I knew switch statements were still
> > expensive due to all the old spectre/meltdown workarounds.
> Wait, are switch-cases also affected? I wasn't aware of that. For sure I
> didn't even consider using ops/indirect calls, but switch-cases... I saw
> lots o'times people replacing indirections with switch-cases, what's the
> point otherwise :D
>
> Thanks,
> Olek

I think it all depends on how the compiler implements them. I know
switch cases used to be done as indirections in some cases. It is
possible they did away with that in the compilers some time ago or
came up with a fix to work around them. I just remember back when that
came up people were going through and replacing switch cases with
if/else blocks in the case of performance sensitive workloads. Odds
are there have been workarounds added to address that, but it is just
something to be aware of.

Thanks,

- Alex