linux-kernel - Re: [PATCH v4 10/10] xen/arm,arm64: enable SWIOTLB

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Date:	Fri, 30 Aug 2013 09:22:10 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
Cc:	Ian Campbell <Ian.Campbell@...rix.com>,
	xen-devel@...ts.xensource.com, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 10/10] xen/arm,arm64: enable SWIOTLB_XEN

On Thu, Aug 29, 2013 at 04:38:17PM +0100, Stefano Stabellini wrote:
> 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?

We do a hypercall for adding devices (See drivers/xen/pci.c). Perhaps
you can piggyback on that one and on ARM return 0 for the devices that
are under Xen control (so IOMMU) and for non-Xen control (those
that will need SWIOTLB) return -EYOUNEEDTOHANDLEIT and use it?
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