On Mon, Oct 15, 2018 at 08:46:41PM +0100, Jean-philippe Brucker wrote: > [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. Hmm. At some level it seems wrong to design hardware interfaces around how Linux happens to probe things. That can change at any time ... > 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 Could you point me at the code you refer to here? -- MST
