[Replying with my personal address because we're having SMTP issues] On 15/10/2018 11:52, Michael S. Tsirkin wrote: > On Fri, Oct 12, 2018 at 02:41:59PM -0500, Bjorn Helgaas wrote: >> s/iommu/IOMMU/ in subject >> >> On Fri, Oct 12, 2018 at 03:59:13PM +0100, Jean-Philippe Brucker wrote: >>> Using the iommu-map binding, endpoints in a given PCI domain can be >>> managed by different IOMMUs. Some virtual machines may allow a subset of >>> endpoints to bypass the IOMMU. In some case the IOMMU itself is presented >> >> s/case/cases/ >> >>> as a PCI endpoint (e.g. AMD IOMMU and virtio-iommu). Currently, when a >>> PCI root complex has an iommu-map property, the driver requires all >>> endpoints to be described by the property. Allow the iommu-map property to >>> have gaps. >> >> I'm not an IOMMU or virtio expert, so it's not obvious to me why it is >> safe to allow devices to bypass the IOMMU. Does this mean a typo in >> iommu-map could inadvertently allow devices to bypass it? > > > Thinking about this comment, I would like to ask: can't the > virtio device indicate the ranges in a portable way? > This would minimize the dependency on dt bindings and ACPI, > enabling support for systems that have neither but do > have virtio e.g. through pci. I thought about adding a PROBE request for this in virtio-iommu, but it wouldn't be usable by a Linux guest because of a bootstrapping problem. Early on, Linux needs a description of device dependencies, to determine in which order to probe them. If the device dependency was described by virtio-iommu itself, the guest could for example initialize a NIC, allocate buffers and start DMA on the physical address space (which aborts if the IOMMU implementation disallows DMA by default), only to find out once the virtio-iommu module is loaded that it needs to cancel all DMA and reconfigure the NIC. With a static description such as iommu-map in DT or ACPI remapping tables, the guest can defer probing of the NIC until the IOMMU is initialized. Thanks, Jean
