On Wed, Aug 27, 2014 at 8:45 AM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > On Wed, Aug 27, 2014 at 08:11:15AM -0700, Andy Lutomirski wrote: >> On Aug 26, 2014 11:46 PM, "Stefan Hajnoczi" <stefanha@xxxxxxxxx> wrote: >> > >> > On Tue, Aug 26, 2014 at 10:16 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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. It wouldn't shock me if someone wants to negotiate this for virtio_mmio some day, but that might be more of a device tree thing. I have no idea how that works, but I think it can wait until someone wants it. I updated the patches, and I'll send them out after I try to test-boot s390 under QEMU :) --Andy _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
