On Wed, Jun 27, 2018 at 07:04:46PM +0100, Jean-Philippe Brucker wrote: > On 26/06/18 19:07, Michael S. Tsirkin wrote: > > So as I pointed out new virtio 0 device isn't really welcome ;) > > Agreed, virtio-iommu is expected to be implemented on virtio 1 and > later. I'll remove the two legacy-related paragraph from the spec and > add a check in the driver as you suggested, to avoid giving the wrong idea. > > > No one bothered implementing virtio 1 in MMIO for all the work > > that was put in defining it. > > That is curious, given that the virtio_mmio driver supports virtio 1 and > from my own experience, porting the MMIO transport to virtio 1 only > requires updating a few registers, when ANY_LAYOUT, modern virtqueue and > status are already implemented. > > > The fact that the MMIO part of the > > spec doesn't seem to allow for transitional devices might > > or might not have something to do with this. > > Sorry, which part do you have in mind? The spec does provide both a > legacy and modern register layout, with version numbers to differentiate > them. Yes but there's no way to implement a transitional virtio mmio device. The version is either "legacy" or "modern". So if you implement a modern device old guests won't work with it. > > So why make it an MMIO device at all? A system with an IOMMU > > will have a PCI bus, won't it? And it's pretty common to > > have the IOMMU be a PCI device on the root bus. > Having the IOMMU outside PCI seems more common though. On Arm and Intel > the IOMMU doesn't have a PCI config space, BARs or capabilities, just a > plain MMIO region and a static number of interrupts. However the AMD > IOMMU appears as a PCI device with additional MMIO registers. I would be > interested in implementing virtio-iommu as a PCI dev at some point, at > least so that we can use MSI-X. > > The problem is that it requires invasive changes in the firmware > interfaces and drivers. They need to describe relationship between IOMMU > and endpoint, and DT or ACPI IORT don't expect the programming interface > to be inside the PCI bus that the IOMMU manages. They don't particularly care IMHO. > Describing it as a > peripheral is more natural. For AMD it is implemented in their driver > using IVHD tables that can't be reused. Right now I don't expect any > success in proposing changes to firmware interfaces or drivers, because > the device is new and paravirtualized, and works out of the box with > MMIO. Hopefully that will change in the future, perhaps when someone > supports DT for AMD IOMMU (they do describe bindings at the end of the > spec, but I don't think it can work in Linux with the existing > infrastructure) That's a bit abstract, I don't really understand the issues involved. ACPI is formatted by QEMU, so firmware does not need to care. And is there even a DT for intel? > Another reason to keep the MMIO transport option is that one > virtio-iommu can manage DMA from endpoints on multiple PCI domains at > the same time, as well as platform devices. Some VMMs might want that, > in which case the IOMMU would be a separate platform device. Which buses are managed by the IOMMU is separate from the bus on which it's programming interface resides. > > Will remove need to bother with dt bindings etc. > That's handled by the firmware drivers and IOMMU layer, there shouldn't > be any special treatment at the virtio layer. In general removal of an > IOMMU needs to be done after removal of all endpoints connected to it, > which should be described using device_link from the driver core. DT or > ACPI is only used to tell drivers where to find resources, and to > describe the DMA topology. > > Thanks, > Jean My point was virtio mmio isn't widely used outside of ARM. Rather than try to make everyone use it, IMHO it's better to start with PCI. -- MST
