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 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
