[Bug 271983] mi_switch hang in KVM on Sapphire Rapids running NET_EPOCH_DRAIN_CALLBACKS

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=271983

--- Comment #10 from R. Christian McDonald <rcm@rcm.sh> ---
I discovered today that it is possible to reproduce this bug on non-Sapphire
Rapids systems. The difference being the use of the legacy vs modern
virtio-rng. 

The legacy virtio-rng doesn't exhibit this problem, it's the modern virtio-rng
that does.

I tested on a non-Sapphire Rapids PVE host this morning and forced the
virtio-rng to explicitly use the modern device
(virtio-rng-pci-non-transitional) instead of the legacy (transitional) device
(which is what PVE sets up by default) and the problem manifests there as well.

Conversely, if I explicitly use the legacy device on the Sapphire Rapids box,
the problem goes away there too.

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