[RFC] virtio: Use DMA MAP API for devices without an IOMMU

[Michael S. Tsirkin]

On Wed, Apr 18, 2018 at 08:47:10AM +0530, Anshuman Khandual wrote:
> On 04/15/2018 05:41 PM, Christoph Hellwig wrote:
> > On Fri, Apr 06, 2018 at 06:37:18PM +1000, Benjamin Herrenschmidt wrote:
> >>>> implemented as DMA API which the virtio core understands. There is no
> >>>> need for an IOMMU to be involved for the device representation in this
> >>>> case IMHO.
> >>>
> >>> This whole virtio translation issue is a mess.  I think we need to
> >>> switch it to the dma API, and then quirk the legacy case to always
> >>> use the direct mapping inside the dma API.
> >>
> >> Fine with using a dma API always on the Linux side, but we do want to
> >> special case virtio still at the arch and qemu side to have a "direct
> >> mapping" mode. Not sure how (special flags on PCI devices) to avoid
> >> actually going through an emulated IOMMU on the qemu side, because that
> >> slows things down, esp. with vhost.
> >>
> >> IE, we can't I think just treat it the same as a physical device.
> > 
> > We should have treated it like a physical device from the start, but
> > that device has unfortunately sailed.
> > 
> > But yes, we'll need a per-device quirk that says 'don't attach an
> > iommu'.
> 
> How about doing it per platform basis as suggested in this RFC through
> an arch specific callback. Because all the virtio devices in the given
> platform would require and exercise this option (to avail bounce buffer
> mechanism for secure guests as an example). So the flag basically is a
> platform specific one not a device specific one.

That's not the case. A single platform can have a mix of virtio and
non-virtio devices. Same applies even within virtio, e.g. the balloon
device always bypasses an iommu.  Further, QEMU supports out of process
devices some of which might bypass the IOMMU.

-- 
MST