interrupt: Make IRQ delivery generic

Because we cannot always assume the irq fd will be the way to send
an IRQ to the guest, this means we cannot make the assumption that
every virtio device implementation should expect an EventFd to
trigger an IRQ.

This commit organizes the code related to virtio devices so that it
now expects a Rust closure instead of a known EventFd. This lets the
caller decide what should be done whenever a device needs to trigger
an interrupt to the guest.

The closure will allow for other type of interrupt mechanism such as
MSI to be implemented. From the device perspective, it could be a
pin based interrupt or an MSI, it does not matter since the device
will simply call into the provided callback, passing the appropriate
Queue as a reference. This design keeps the device model generic.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

vm-virtio/src/device.rs

@@ -13,6 +13,8 @@ use std::sync::Arc;
 use vm_memory::GuestMemoryMmap;
 use vmm_sys_util::EventFd;
 
+pub type VirtioInterrupt = Box<Fn(&Queue) -> std::result::Result<(), std::io::Error> + Send + Sync>;
+
 /// Trait for virtio devices to be driven by a virtio transport.
 ///
 /// The lifecycle of a virtio device is to be moved to a virtio transport, which will then query the
@@ -46,7 +48,7 @@ pub trait VirtioDevice: Send {
     fn activate(
         &mut self,
         mem: GuestMemoryMmap,
-        interrupt_evt: EventFd,
+        interrupt_evt: Arc<VirtioInterrupt>,
         status: Arc<AtomicUsize>,
         queues: Vec<Queue>,
         queue_evts: Vec<EventFd>,
@@ -54,7 +56,7 @@ pub trait VirtioDevice: Send {
 
     /// Optionally deactivates this device and returns ownership of the guest memory map, interrupt
     /// event, and queue events.
-    fn reset(&mut self) -> Option<(EventFd, Vec<EventFd>)> {
+    fn reset(&mut self) -> Option<(Arc<VirtioInterrupt>, Vec<EventFd>)> {
         None
     }