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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 6 Dec 2018 10:51:33 -0800
Message-ID: <CAHk-=wh=aLuvLu++ZcehjDLgZfSqOVK+whecEq_-JP12VQ96sg@mail.gmail.com>
Subject: Re: [RFC] avoid indirect calls for DMA direct mappings
To:     Christoph Hellwig <hch@lst.de>
Cc:     iommu@lists.linux-foundation.org, brouer@redhat.com,
        tariqt@mellanox.com, ilias.apalodimas@linaro.org, toke@toke.dk,
        robin.murphy@arm.com,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>
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On Thu, Dec 6, 2018 at 10:43 AM Christoph Hellwig <hch@lst.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