netdev - Re: the XSK buffer pool needs be to reverted

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Date:   Sat, 27 Jun 2020 09:02:36 +0200
From:   Christoph Hellwig <hch@....de>
To:     Jonathan Lemon <jonathan.lemon@...il.com>
Cc:     Christoph Hellwig <hch@....de>,
        Björn Töpel <bjorn.topel@...el.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        netdev@...r.kernel.org, iommu@...ts.linux-foundation.org
Subject: Re: the XSK buffer pool needs be to reverted

On Fri, Jun 26, 2020 at 01:54:12PM -0700, Jonathan Lemon wrote:
> On Fri, Jun 26, 2020 at 09:47:25AM +0200, Christoph Hellwig wrote:
> >
> > Note that this is somewhat urgent, as various of the APIs that the code
> > is abusing are slated to go away for Linux 5.9, so this addition comes
> > at a really bad time.
> 
> Could you elaborate on what is upcoming here?

Moving all these calls out of line, and adding a bypass flag to avoid
the indirect function call for IOMMUs in direct mapped mode.

> Also, on a semi-related note, are there limitations on how many pages
> can be left mapped by the iommu?  Some of the page pool work involves
> leaving the pages mapped instead of constantly mapping/unmapping them.

There are, but I think for all modern IOMMUs they are so big that they
don't matter.  Maintaines of the individual IOMMU drivers might know
more.

> On a heavily loaded box with iommu enabled, it seems that quite often
> there is contention on the iova_lock.  Are there known issues in this
> area?

I'll have to defer to the IOMMU maintainers, and for that you'll need
to say what code you are using.  Current mainlaine doesn't even have
an iova_lock anywhere.