Bad things happen when the VIRTIO_RING_F_EVENT_IDX feature isn't
negotiated and we try to write the avail_event anyway. SeaBIOS, for
example, stores internal data where avail_event should be [1].

Technically the Virtio specification doesn't forbid the device from
writing the avail_event, and it's up to the driver to reserve space for it
("the transitional driver [...] MUST allocate the total number of bytes
for the virtqueue according to [formula containing the avail event]").

But it doesn't hurt us to avoid writing avail_event, and kvmtool needs
changes for interrupt suppression anyway, in order to comply with the
spec. Indeed Virtio 1.0 cs04 says, in 2.4.7.2 Device Requirements:
Virtqueue Interrupt Suppression:
"""
If the VIRTIO_F_EVENT_IDX feature bit is not negotiated:
* The device MUST ignore the used_event value.
* After the device writes a descriptor index into the used ring:
  - If flags is 1, the device SHOULD NOT send an interrupt.
"""

So let's do that.

[1] https://patchwork.kernel.org/patch/10038931/



Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>