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

From: Michael S. Tsirkin
Date: Wed Oct 28 2015 - 07:35:58 EST

On Wed, Oct 28, 2015 at 05:09:47PM +0900, David Woodhouse wrote:
> 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* â 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

Well APIs are just that - internal kernel APIs.
If the only user of an API is virtio, we can strick the
code in virtio.h just as well.
I think controlling this dynamically and not statically
in e.g. devicetree is important though.

E.g. on intel x86, there's an option iommu=pt which does the 1:1
thing for devices when used by kernel, but enables
the iommu if used by userspace/VMs.

Something like this would be needed for other platforms IMHO.

And given that
1. virtio seems the only user so far
2. supporting this per device seems like something that
might become useful in the future
maybe we'd better make this part of virtio transports.

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