On Thu, Sep 26, 2024 at 11:02:37PM -0700, Nicolin Chen wrote: > On Fri, Sep 27, 2024 at 01:38:08PM +0800, Yi Liu wrote: > > > > Does it mean each vIOMMU of VM can only have > > > > one s2 HWPT? > > > > > > Giving some examples here: > > > - If a VM has 1 vIOMMU, there will be 1 vIOMMU object in the > > > kernel holding one S2 HWPT. > > > - If a VM has 2 vIOMMUs, there will be 2 vIOMMU objects in the > > > kernel that can hold two different S2 HWPTs, or share one S2 > > > HWPT (saving memory). > > > > So if you have two devices assigned to a VM, then you may have two > > vIOMMUs or one vIOMMU exposed to guest. This depends on whether the two > > devices are behind the same physical IOMMU. If it's two vIOMMUs, the two > > can share the s2 hwpt if their physical IOMMU is compatible. is it? > > Yes. > > > To achieve the above, you need to know if the physical IOMMUs of the > > assigned devices, hence be able to tell if physical IOMMUs are the > > same and if they are compatible. How would userspace know such infos? > > My draft implementation with QEMU does something like this: > - List all viommu-matched iommu nodes under /sys/class/iommu: LINKs > - Get PCI device's /sys/bus/pci/devices/0000:00:00.0/iommu: LINK0 > - Compare the LINK0 against the LINKs > > We so far don't have an ID for physical IOMMU instance, which can > be an alternative to return via the hw_info call, otherwise. We could return the sys/class/iommu string from some get_info or something > For compatibility to share a stage-2 HWPT, basically we would do > a device attach to one of the stage-2 HWPT from the list that VMM > should keep. This attach has all the compatibility test, down to > the IOMMU driver. If it fails, just allocate a new stage-2 HWPT. Ideally just creating the viommu should validate the passed in hwpt is compatible without attaching. Jason
