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. -- MST -- To unsubscribe from this list: send the line "unsubscribe linux-s390" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
