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Date: Wed, 28 Aug 2013 21:07:52 +0100
From: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
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To: Ian Campbell <ian.campbell@citrix.com>
CC: Stefano Stabellini <stefano.stabellini@eu.citrix.com>,
        <xen-devel@lists.xensource.com>, <linux-kernel@vger.kernel.org>,
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Subject: Re: [PATCH v4 10/10] xen/arm,arm64: enable SWIOTLB_XEN
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On Thu, 15 Aug 2013, Ian Campbell wrote:
> On Thu, 2013-08-15 at 12:10 +0100, Stefano Stabellini wrote:
> > At the moment always rely on swiotlb-xen, but when Xen starts supporting
> > hardware IOMMUs we'll be able to avoid it conditionally on the presence
> > of an IOMMU on the platform.
> 
> Do we have any idea how we are going to do this?
> 
> It's extra complicated if you consider that on some systems on some of
> the devices are behind an IOMMU :-/
> 
> I wonder if we can enumerate which devices have an IOMMU at boot time
> and force a ludicrous dma mask (such as 0) if one isn't present in order
> to force us to always take the exchange_and_pin path?

We don't need to worry about how to specify which devices need to go via
the swiotlb internally, because we have our own arm specific
dma_map_ops. At the moment they are just implemented using the
swiotlb-xen functions, but we could easily provide wrappers that check
our own internal whitelist/blacklist and go via swiotlb-xen only in
those cases.