[Devel] [PATCH hci-8.1 1/2] vfio: take the kvm reference only for drivers that read device->kvm

[no-reply at sashiko.virtuozzo.com]

> A device takes a reference to its group's kvm at the first open and
> drops it at the last close, so that device->kvm stays valid for the
> driver in between. QEMU's cpr-exec and cpr-transfer keep the device
> open while the VM is re-created, so the first VM stays referenced for
> as long as the device is: its vcpus, mm and in-kernel PIT thread with
> it. Seen as a second KVM instance in debugfs after a live update of a
> vGPU VM.
> 
> Only three drivers read device->kvm after open_device(): KVMGT,
> vfio_ap and vfio-pci's zdev support. Let them say so with
> needs_kvm_ref, and take no reference for the others, which then can't
> keep a VM alive. Tested with nvidia-vgpu-vfio 595.71.03 on RTX 6000
> Ada SR-IOV VFs, a vfio-pci variant driver that doesn't read the
> pointer.
> 
> Fixes: 2b48f52f2bff ("vfio: fix deadlock between group lock and kvm lock")
> https://virtuozzo.atlassian.net/browse/VSTOR-150597
> Feature: fix ms/vfio
> 
> Assisted-by: LLM
> Signed-off-by: Andrey Drobyshev <andrey.drobyshev at virtuozzo.com>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.vzint.dev/#/patchset/20261105121733.643754-1-andrey.drobyshev@virtuozzo.com?part=1