virtio_pci_modern: enable virtqueues before completing attachHEADmain

Most of our drivers setup interrupt(s) and virtqueues are enabled at
that time, but nothing will currently enable them for devices that we're
only going to poll.  This most notably affects virtio-random, and the
problem can be observed with qemu's q35 emulation and a random device
attached.  The q35 implementation ignores the ring unless it's enabled,
so vtrnd_harvest() will consistently EAGAIN rather than contribute
anything of value, but with this patch it becomes functional.

This can be reproduced with a qemu invocation like so:

qemu-system-x86_64 -machine q35,accel=tcg -cpu max -smp 2 -m 2G \
  -drive file=FreeBSD-16.0-CURRENT-amd64-ufs.qcow2,if=virtio \
  -device pcie-root-port,id=rp1,bus=pcie.0,chassis=1 \
  -object rng-random,id=rng0,filename=/dev/urandom \
  -device virtio-rng-pci,rng=rng0,bus=rp1 \
  -display curses

Reviewed by:	bnovkov, kfv
Sponsored by:	Stormshield
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D61052

0 files changed, 0 insertions, 0 deletions