Re: [PATCH 0/3] virtio: Clean up scatterlists and use the DMA API

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Aug 27, 2014 at 08:11:15AM -0700, Andy Lutomirski wrote:
> On Aug 26, 2014 11:46 PM, "Stefan Hajnoczi" <stefanha@gmail.com> wrote:
> >
> > On Tue, Aug 26, 2014 at 10:16 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> > > There are two outstanding issues.  virtio_net warns if DMA debugging
> > > is on because it does DMA from the stack.  (The warning is correct.)
> > > This also is likely to do something unpleasant to s390.
> > > (Maintainers are cc'd -- I don't know what to do about it.)
> >
> > This changes the semantics of vring and breaks existing guests when
> > bus address != physical address.
> >
> > Can you use a transport feature bit to indicate that bus addresses are
> > used?  That way both approaches can be supported.
> 
> I can try to support both styles of addressing, but I don't think that
> this can be negotiated between the device (i.e. host or physical
> virtio-speaking device) and the guest.  In the Xen case that I care
> about (Linux on Xen on KVM), the host doesn't know about the
> translation at all -- Xen is an intermediate layer that only the guest
> is aware of.  In this case, there are effectively two layers of
> virtualization, and only the inner one (Xen) knows about the
> translation despite the fact that the the outer layer is the one
> providing the virtio device.
> 
> I could change virtio_ring to use the DMA API only if requested by the
> lower driver (virtio_pci, etc) and to have only virtio_pci enable that
> feature.  Will that work for all cases?
> 
> On s390, this shouldn't work just like the current code.  On x86, I
> think that if QEMU ever starts exposing an IOMMU attached to a
> virtio-pci device, then QEMU should expect that IOMMU to be used.  If
> QEMU expects to see physical addresses, then it shouldn't advertise an
> IOMMU.  Since QEMU doesn't currently support guest IOMMUs, this should
> be fine for everything that uses QEMU.
> 
> At least x86's implementation of the DMA ops for devices that aren't
> behind an IOMMU should be very fast.
> 
> Are there any other weird cases for which this might be a problem?
> 
> >
> > Please also update the virtio specification:
> > https://tools.oasis-open.org/version-control/browse/wsvn/virtio/
> >
> 
> I'm not sure it will need an update.  Perhaps a note in the PCI
> section indicating that, if the host expects the guest to program an
> IOMMU, then it should use the appropriate platform-specific mechanism
> to expose that IOMMU.
> 
> --Andy

If there's no virtio mechanism to negotate enabling/disabling
translations, then specification does not need an extension.

-- 
MST