linux-kernel - Re: [RFC] avoid indirect calls for DMA direct mappings

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Date:   Thu, 6 Dec 2018 10:51:33 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Christoph Hellwig <hch@....de>
Cc:     iommu@...ts.linux-foundation.org, brouer@...hat.com,
        tariqt@...lanox.com, ilias.apalodimas@...aro.org, toke@...e.dk,
        robin.murphy@....com,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] avoid indirect calls for DMA direct mappings

On Thu, Dec 6, 2018 at 10:43 AM Christoph Hellwig <hch@....de> wrote:
>
> >
> >  (a) the dma_is_direct() function/macro uses "likely()" for the test
>
> I'm a little worried about that.  Yes, for the common case it is
> likely.  But if you run a setup where you say always have an iommu
> it is not, in fact it is never called in that case, but we only know
> that at runtime.

Note that "likely()" doesn't have any really huge overhead - it just
makes the compiler move the unlikely case out-of-line.

Compared to the overhead of the indirect branch, it's simply not a
huge deal, it's more a mispredict and cache layout issue.

So marking something "likely()" when it isn't doesn't really penalize
things too much. It's not like an exception or anything like that,
it's really just a marker for better code layout.

                  Linus