On Tue, Sep 2, 2014 at 1:53 PM, Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx> wrote: > On Mon, 2014-09-01 at 22:55 -0700, Andy Lutomirski wrote: >> >> On x86, at least, I doubt that we'll ever see a physically addressed >> PCI virtio device for which ACPI advertises an IOMMU, since any sane >> hypervisor will just not advertise an IOMMU for the virtio device. >> But are there arm64 or PPC guests that use virtio_pci, that have >> IOMMUs, and that will malfunction if the virtio_pci driver ends up >> using the IOMMU? I certainly hope not, since these systems might be >> very hard-pressed to work right if someone plugged in a physical >> virtio-speaking PCI device. > > It will definitely not work on ppc64. We always have IOMMUs on pseries, > all PCI busses do, and because it's a paravirtualized environment, > napping/unmapping pages means hypercalls -> expensive. > > But our virtio implementation bypasses it in qemu, so if virtio-pci > starts using the DMA mapping API without changing the DMA ops under the > hood, it will break for us. > Let's take a step back from from the implementation. What is a driver for a virtio PCI device (i.e. a PCI device with vendor 0x1af4) supposed to do on ppc64? It can send the device physical addresses and ignore the normal PCI DMA semantics, which is what the current virtio_pci driver does. This seems like a layering violation, and this won't work if the device is a real PCI device. Alternatively, it can treat the device like any other PCI device and use the IOMMU. This is a bit slower, and it is also incompatible with current hypervisors. There really are virtio devices that are pieces of silicon and not figments of a hypervisor's imagination [1]. We could teach virtio_pci to use physical addressing on ppc64, but that seems like a pretty awful hack, and it'll start needing quirks as soon as someone tries to plug a virtio-speaking PCI card into a ppc64 machine. Ideas? x86 and arm seem to be safe here, since AFAIK there is no such thing as a physically addressed virtio "PCI" device on a bus with an IOMMU on x86, arm, or arm64. [1] https://lwn.net/Articles/580186/ > Cheers, > Ben. > > -- Andy Lutomirski AMA Capital Management, LLC _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
