https://github.com/virtio-win/kvm-guest-drivers-windows/issues/1312

Installation of build 266 (and later) hangs on 2025 (with PollMode enabled)

The problem is reproducible 100% on upstream user (Alibaba Cloud), they can't reproduce it on qemu. 

Looking at guest dump from frozen machine I do not see any special reason why this can't happen on qemu, there is a difference with typical behavior on qemu setup, but the root cause of the difference is not clear and the flow in the driver is definitely problematic.

2 CPUs, 2 queues

On customer setup: 

Windows configures RSS to work only on CPU0, so both poll handlers have the same affinity of CPU0.  But the interrupts naturally may come on any of 2 CPUs , if the interrupt comes on Q1 this triggers poll handler 1 on CPU0, but the packet is related to Q0, so we trigger poll handler 0 and the system calls it synchronously because this poll handler's affinity is also CPU0.

On typical setup:

Windows configures RSS to work both CPUs, 2 poll handlers has different affinities and no problem happens.  

Reproduction procedure worked for me:

1. Configure virtio device with 2 queues, 2 cpus 
2. Install driver 266... 274 for Win11/2025
3. Open Powershell admin prompt
4. Run get-netadapter to see name(s) of virtio-net adapter
5. Run get-NetAdapterRss -Name "adapter name"
6. If printed MaxProcessors = 1, goto 8
7. Run set-NetAadapterRss -Name "adapter name" -MaxProcessors 1
8. If the OS is not frozen yet, try to turn the link off-on (via set_link hmp, for example) or ping the virtio-net from several different LAN addresses.