On Thu, Feb 06, 2025 at 02:22:01PM -0400, Jason Gunthorpe wrote: > On Thu, Feb 06, 2025 at 06:18:14PM +0000, Shameerali Kolothum Thodi wrote: > > > > So even if you invent an iommu ID we cannot accept it as a handle to > > > create viommu in iommufd. > > > > Creating the vIOMMU only happens when the user does a cold/hot plug of > > a VFIO device. At that time Qemu checks whether the assigned id matches > > with whatever the kernel tell it. > > This is not hard up until the guest is started. If you boot a guest > without a backing viommu iommufd object then there will be some more > complexities.
Yea, I imagined that things would be complicated with hotplugs.. On one hand, I got the part that we need some fixed link forehand to ease migration/hotplugs. On the other hand, all IOMMUFD ioctls need a VFIO device FD, which brings the immediate attention that we cannot even decide vSMMU's capabilities being reflected in its IDR/IIDR registers, without a coldplug device -- if we boot a VM (one vSMMU<->pSMMU) with only a hotplug device, the IOMMU_GET_HW_INFO cannot be done during guest kernel probing vSMMU instance. So we would have to reset the vSMMU "HW" after the device hotplug? Nicolin
