Two issues with virtio device reset:

1. Karl Mehltretter reported that virtio_reset_device() promises
   callbacks are not in progress after reset, but only PCI transports
   actually synchronize callbacks - other transports leave a window
   where a handler already executing keeps running while the driver
   tears down state.

2. sashiko reported a race in virtio_pci_modern: the avq interrupt
   handler calls virtqueue_get_buf concurrently with
   virtqueue_detach_unused_buf in vp_modern_avq_cleanup, and there
   is no synchronize_irq between reset and cleanup.

Fix 1 by adding virtio_synchronize_cbs in the core after reset,
then dropping the now-redundant per-transport sync calls. Fix 2 by
moving avq cleanup from vp_reset to a modern-specific del_vqs
wrapper, which runs after callbacks have been synchronized - and is
where buffer teardown conceptually belongs.

Changes v3->v4:
    patch 1: add Tested-by and Acked-by from Karl
    patch 2: was patch 3 in v3; instead of moving
        vp_modern_avq_cleanup() to the common vp_del_vqs(),
        add a vp_modern_del_vqs() wrapper in
        virtio_pci_modern.c. Split out callback sync removal
        into a separate patch.
    patch 3: was patch 2 in v3 (legacy only); now includes
        modern transport too.

Changes v2->v3:
    patch 1: unchanged
    patch 2: split from v2 patch 2 - legacy part only
    patch 3: new in v3

Michael S. Tsirkin (3):
  virtio: synchronize callbacks after device reset
  virtio_pci_modern: move avq cleanup from reset to del_vqs
  virtio_pci: drop callback sync on reset

 drivers/virtio/virtio.c            |  2 ++
 drivers/virtio/virtio_pci_legacy.c |  2 --
 drivers/virtio/virtio_pci_modern.c | 15 ++++++++-------
 3 files changed, 10 insertions(+), 9 deletions(-)

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MST