From: Yu Zhang <[email protected]> Sent: Friday, May 15, 2026 9:54 AM > > On Fri, May 15, 2026 at 02:51:38PM +0000, Michael Kelley wrote: > > From: Yu Zhang <[email protected]> Sent: Friday, May 15, 2026 > > 7:00 AM > > > > > > On Thu, May 14, 2026 at 06:13:24PM +0000, Michael Kelley wrote: > > > > From: Yu Zhang <[email protected]> Sent: Monday, May 11, > > > > 2026 9:24 AM
[....] > > > > > > > > Previous versions of this function did hv_iommu_detach_dev(). With that > > > > call > > > > removed from here, hv_iommu_detach_dev() is only called when attaching a > > > > domain to a device that already has a domain attached. Is it the case > > > > that > > > > Hyper-V doesn't require the detach as a cleanup step? > > > > > > > > > > The IOMMU core attaches the device to release_domain (our blocking domain) > > > before calling release_device(), so I believe the explicit detach in the > > > RFC > > > was redundant. I simply didn't realize that at the time. > > > > > > > Got it. But after the IOMMU core attaches the device to the blocking > > domain, there's the possibility that the vPCI device is rescinded by > > Hyper-V and it goes away entirely. Or the device might be subjected > > to an "unbind/bind" cycle in Linux. Does the detach need to be done > > on the blocking domain in such cases? In this version of the patches, the > > Hyper-V "attach" and "detach" hypercalls still end up unbalanced. That > > seems a bit untidy at best, and I wonder if there are scenarios where > > Hyper-V will complain about the lack of balance. > > > > Thank you, Michael. May I ask what "the vPCI device is rescinded by > Hyper-V and it goes away entirely" mean? > See the documentation at Documentation/virt/hyperv/vpci.rst in a kernel source code tree, and particularly the section entitled "PCI Device Removal". Such removals can and do happen in running Azure guest VMs. Start with that info and then I'll do my best to answer follow-up questions you may have. The unbind/bind case is separate, but has some of the same effects in that Linux should be removing all setup of the PCI device. There's actually two unbind steps -- one to unbind the device-specific driver (e.g., the Mellanox MLX5 driver or the NMVe driver) driver from the device, and potentially a second to unbind the VMBus vPCI driver from the device. These unbind/bind sequences can be done in the Linux guest without the Hyper-V host rescinding the device. Michael
