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Date: Thu, 29 Aug 2013 16:38:17 +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, 29 Aug 2013, Ian Campbell wrote:
> On Wed, 2013-08-28 at 21:07 +0100, Stefano Stabellini wrote:
> > 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.
> 
> OK, but how do we decide which devices go on those lists? We need some
> sort of indication from the hypervisor, don't we? Only Xen knows if it
> has an iommu it can use because the iommu must necessarily be hidden
> from Linux.

Right. Maybe Xen could mark the devices that are safe to use without
swiotlb-xen on device tree? Adding a property to the node?