virtio-devices: Simplify interrupt handling

Previously the interrupt was created in VirtioPciDevice, moved via the
Option::take() to the VirtioPciDeviceActivator and then moved to the
VirtioDevice upon activation. On reset it would be moved back ready for
reactivation.

Since this already an Arc type remove the wrapping Option and instead
refcount it such that the VirtioPciDevice can continue to hold onto it
for later activations.

This significantly simplifies the reset() logic as there is no need to
hand back the interrupt.

A few devices used whether the interrupt was Some to make triggering an
interrupt a no-op. However the MSI-X interrupt routing already drops the
interrupt if the driver hasn't yet configured the vector so it is safe
to trigger the interrupt before device activation (e.g. balloon resize
request before driver loaded).

VirtioCommon still retains an Option<..> for the interrupt as the
interrupt is not known until activation time (after this has been
created). A helper VirtioCommon::trigger_interrupt() has been added to
handle this.

Signed-off-by: Rob Bradford <rbradford@meta.com>

virtio-devices/src/device.rs

@@ -123,11 +123,8 @@ pub trait VirtioDevice: Send {
     /// Activates this device for real usage.
     fn activate(&mut self, context: ActivationContext) -> ActivateResult;
 
-    /// Optionally deactivates this device and returns ownership of the guest memory map, interrupt
-    /// event, and queue events.
-    fn reset(&mut self) -> Option<Arc<dyn VirtioInterrupt>> {
-        None
-    }
+    /// Optionally deactivates this device.
+    fn reset(&mut self) {}
 
     /// Returns the list of shared memory regions required by the device.
     fn get_shm_regions(&self) -> Option<VirtioSharedMemoryList> {
@@ -290,7 +287,7 @@ impl VirtioCommon {
         Ok(())
     }
 
-    pub fn reset(&mut self) -> Option<Arc<dyn VirtioInterrupt>> {
+    pub fn reset(&mut self) {
         self.queue_evts.clear();
 
         // Resume the virtio thread if it was paused. Reset must always
@@ -315,8 +312,16 @@ impl VirtioCommon {
             }
         }
 
-        // Return the interrupt
-        Some(self.interrupt_cb.take().unwrap())
+        // Drop the interrupt callback clone
+        self.interrupt_cb = None;
+    }
+
+    pub fn trigger_interrupt(&self, int_type: VirtioInterruptType) -> std::io::Result<()> {
+        if let Some(interrupt_cb) = &self.interrupt_cb {
+            interrupt_cb.trigger(int_type)
+        } else {
+            Ok(())
+        }
     }
 
     // Wait for the worker thread to finish and return
@@ -406,12 +411,9 @@ impl Pausable for VirtioCommon {
         }
 
         // Also trigger interrupts into the guest to wake up the driver to avoid a "livelock"
-        if let Some(interrupt_cb) = &self.interrupt_cb {
-            for i in 0..self.queue_evts.len() {
-                interrupt_cb
-                    .trigger(crate::VirtioInterruptType::Queue(i as u16))
-                    .ok();
-            }
+        for i in 0..self.queue_evts.len() {
+            self.trigger_interrupt(crate::VirtioInterruptType::Queue(i as u16))
+                .ok();
         }
 
         Ok(())