On 1/29/25 4:04 PM, Jason Gunthorpe wrote: > On Wed, Jan 29, 2025 at 03:54:48PM +0100, Eric Auger wrote: >>>> or you are just mentioning it here because >>>> it is still possible to make use of that. I think from previous discussions the >>>> argument was to adopt a more dedicated MSI pass-through model which I >>>> think is approach-2 here. >>> The basic flow of the pass through model is shown in the last two >>> patches, it is not fully complete but is testable. It assumes a single >>> ITS page. The VM would use IOMMU_OPTION_SW_MSI_START/SIZE to put the >>> ITS page at the correct S2 location and then describe it in the ACPI >>> as an ITS page not a RMR. >> This is a nice to have feature but not mandated in the first place, >> is it? > Not mandated. It just sort of happens because of the design. IMHO > nothing should use it because there is no way for userspace to > discover how many ITS pages there may be. > >>> This missing peice is cleaning up the ITS mapping to allow for >>> multiple ITS pages. I've imagined that kvm would someone give iommufd >>> a FD that holds the specific ITS pages instead of the >>> IOMMU_OPTION_SW_MSI_START/SIZE flow. >> That's what I don't get: at the moment you only pass the gIOVA. With >> technique 2, how can you build the nested mapping, ie. >> >> S1 S2 >> gIOVA -> gDB -> hDB >> >> without passing the full gIOVA/gDB S1 mapping to the host? > The nested S2 mapping is already setup before the VM boots: > > - The VMM puts the ITS page (hDB) into the S2 at a fixed address (gDB) Ah OK. Your gDB has nothing to do with the actual S1 guest gDB, right? It is computed in iommufd_sw_msi_get_map() from the sw_msi_start pool. Is that correct? In https://lore.kernel.org/all/20210411111228.14386-9-eric.auger@xxxxxxxxxx/ I was passing both the gIOVA and the "true" gDB Eric > - The ACPI tells the VM that the GIC has an ITS page at the S2's > address (hDB) > - The VM sets up its S1 with a gIOVA that points to the S2's ITS > page (gDB). The S2 already has gDB -> hDB. > - The VMM traps the gIOVA write to the MSI-X table. Both the S1 and > S2 are populated at this moment. > > If you have multiple ITS pages then the ACPI has to tell the guest GIC > about them, what their gDB address is, and what devices use which ITS. > > Jason >
