virtio-PCI interrupt corner cases

[Demi Marie Obenour <demiobenour@gmail.com>]

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There are several corner cases in virtio-PCI interrupt handling.
I'm trying to figure out what the expected behavior is in these cases,
as the spec isn't clear.

1. Suppose virtqueue 0 is mapped to MSI-X vector 5.  The device
   triggers an interrupt on virtqueue 0.  Vector 5 is currently masked,
   so the interrupt becomes pending.  The driver then map virtqueue
   0 to vector 6 and this succeeds.

   a. Is there still have an interrupt pending on vector 5?
   b. If vector 6 is unmasked, is an interrupt delivered immediately?
   c. If vector 6 is masked, does it become pending?

2. Suppose virtqueue 1 is mapped to MSI-X vector 7.  The device
   triggers an interrupt on virtqueue 1.  Vector 7 is currently masked,
   so the interrupt becomes pending.  The driver then maps virtqueue 1
   to NO_VECTOR.

   Is there still an interrupt pending on vector 7, or is the interrupt
   lost?

3. Suppose both virtqueues 3 and 4 are mapped to MSI-X vector 3.
   The device triggers interrupts on both virtqueues.  Does the driver
   receive one interrupt or two?

I don't have access to the PCI specification (paywall).
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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