Re: [PATCH v3 0/3] virtio DMA API core stuff

From: David Woodhouse <dwmw2@infradead.org>
To: Christian Borntraeger <borntraeger@de.ibm.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Andy Lutomirski <luto@kernel.org>
Cc: linux-s390 <linux-s390@vger.kernel.org>,
	Joerg Roedel <jroedel@suse.de>, KVM <kvm@vger.kernel.org>,
	benh@kernel.crashing.org,
	Sebastian Ott <sebott@linux.vnet.ibm.com>,
	linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	Martin Schwidefsky <schwidefsky@de.ibm.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v3 0/3] virtio DMA API core stuff
Date: Wed, 28 Oct 2015 17:09:47 +0900	[thread overview]
Message-ID: <1446019787.3405.203.camel@infradead.org> (raw)
In-Reply-To: <56307BD1.6010806@de.ibm.com>

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On Wed, 2015-10-28 at 16:40 +0900, Christian Borntraeger wrote:
> Am 28.10.2015 um 16:17 schrieb Michael S. Tsirkin:
> > On Tue, Oct 27, 2015 at 11:38:57PM -0700, Andy Lutomirski wrote:
> > > This switches virtio to use the DMA API unconditionally.  I'm sure
> > > it breaks things, but it seems to work on x86 using virtio-pci, with
> > > and without Xen, and using both the modern 1.0 variant and the
> > > legacy variant.
> > 
> > I'm very glad to see work on this making progress.
> > 
> > I suspect we'll have to find a way to make this optional though, and
> > keep doing the non-DMA API thing with old devices.  And I've been
> > debating with myself whether a pci specific thing or a feature bit is
> > preferable.
> > 
> 
> We have discussed that at kernel summit. I will try to implement a dummy dma_ops for
> s390 that does 1:1 mapping and Ben will look into doing some quirk to handle "old"
> code in addition to also make it possible to mark devices as iommu bypass (IIRC,
> via device tree, Ben?)

Right. You never eschew the DMA API in the *driver* — you just expect
the DMA API to do the right thing for devices which don't need
translation (with platforms using per-device dma_ops and generally
getting their act together).

We're pushing that on the platforms where it's currently an issue,
including Power, SPARC and S390.

-- 
dwmw2

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