virtio-devices: make ioeventfds() return an iterator

MSHV's SEV-SNP implementation calls ioeventfds whenever there is an
event.

This change removes the need frequent allocation and deallocation of a
vector, while at the same time makes sure other call sites are
unaffected.

Signed-off-by: Wei Liu <liuwe@microsoft.com>

virtio-devices/src/transport/mod.rs

@@ -9,5 +9,5 @@ pub use pci_common_config::{VirtioPciCommonConfig, VIRTIO_PCI_COMMON_CONFIG_ID};
 pub use pci_device::{VirtioPciDevice, VirtioPciDeviceActivator, VirtioPciDeviceError};
 
 pub trait VirtioTransport {
-    fn ioeventfds(&self, base_addr: u64) -> Vec<(&EventFd, u64)>;
+    fn ioeventfds(&self, base_addr: u64) -> impl Iterator<Item = (&EventFd, u64)>;
 }

virtio-devices/src/transport/pci_device.rs

@@ -830,18 +830,14 @@ impl VirtioPciDevice {
 }
 
 impl VirtioTransport for VirtioPciDevice {
-    fn ioeventfds(&self, base_addr: u64) -> Vec<(&EventFd, u64)> {
+    fn ioeventfds(&self, base_addr: u64) -> impl Iterator<Item = (&EventFd, u64)> {
         let notify_base = base_addr + NOTIFICATION_BAR_OFFSET;
-        self.queue_evts()
+        self.queue_evts().iter().enumerate().map(move |(i, event)| {
-            .iter()
+            (
-            .enumerate()
+                event,
-            .map(|(i, event)| {
+                notify_base + i as u64 * u64::from(NOTIFY_OFF_MULTIPLIER),
-                (
+            )
-                    event,
+        })
-                    notify_base + i as u64 * u64::from(NOTIFY_OFF_MULTIPLIER),
-                )
-            })
-            .collect()
     }
 }