virtio-devices: Implement virtio-device rtc

This change will allow us to get accurate time over ptp in guests
started from a MSHV-virtualized Linux host. Implementing it as a
virtio device is preferable to using the existing kvm_ptp because:

kvm_ptp relies on hypercalls that only exist on host kernels running
kvm. Virtio-rtc gives us more flexibility in what clock types we want
to provide. We can later extend the device to implement multiple clocks
(smeared UTC, TAI, monotonic, etc.). Virtio-rtc protocol supports
alarms. Alarms may later enable usecases where the guests can do their
own VM lifecycle management without relying on a host-side
orchestrator.

Implement device backend for virtio-rtc. Currently this implementation
encompasses:

1. CONFIG, CAP, READ, CROSSCAP (returns false)
2. One PTP clock is presented of type
VIRTIO_RTC_CLOCK_UTC_MAYBE_SMEARED with leap_second_smearing
VIRTIO_RTC_SMEAR_UNSPECIFIED

The device is disabled by default, requiring --rtc to be passed

Not implemented but theoretically supported by virtio-rtc is:

1. Cross-timestamping support
2. The alarm queue

Fixes #7730

Signed-off-by: Cameron Baird <cameronbaird@microsoft.com>

vmm/src/device_manager.rs

@@ -139,6 +139,7 @@ const DEBUGCON_DEVICE_NAME: &str = "__debug_console";
 #[cfg(target_arch = "aarch64")]
 const GPIO_DEVICE_NAME: &str = "__gpio";
 const RNG_DEVICE_NAME: &str = "__rng";
+const RTC_DEVICE_NAME: &str = "__rtc";
 const IOMMU_DEVICE_NAME: &str = "__iommu";
 #[cfg(feature = "pvmemcontrol")]
 const PVMEMCONTROL_DEVICE_NAME: &str = "__pvmemcontrol";
@@ -193,6 +194,10 @@ pub enum DeviceManagerError {
     #[error("Cannot create virtio-rng device")]
     CreateVirtioRng(#[source] io::Error),
 
+    /// Cannot create virtio-rtc device
+    #[error("Cannot create virtio-rtc device")]
+    CreateVirtioRtc(#[source] io::Error),
+
     /// Cannot create generic vhost-user device
     #[error("Cannot create generic vhost-user device")]
     CreateGenericVhostUser(#[source] virtio_devices::vhost_user::Error),
@@ -2654,6 +2659,9 @@ impl DeviceManager {
         // Add vDPA devices if required
         self.make_vdpa_devices(snapshot)?;
 
+        // Add virtio-rtc device
+        self.make_virtio_rtc_devices(snapshot)?;
+
         Ok(())
     }
     /// Creates a [`MetaVirtioDevice`] from the provided [`DiskConfig`].
@@ -3116,6 +3124,53 @@ impl DeviceManager {
         Ok(())
     }
 
+    fn make_virtio_rtc_devices(&mut self, snapshot: Option<&Snapshot>) -> DeviceManagerResult<()> {
+        let Some(mut rtc_config) = self.config.lock().unwrap().rtc.clone() else {
+            return Ok(());
+        };
+
+        info!("Creating virtio-rtc device: {rtc_config:?}");
+
+        let id = match rtc_config.pci_common.id.as_ref() {
+            Some(id) => id.clone(),
+            None => rtc_config
+                .pci_common
+                .id
+                .insert(RTC_DEVICE_NAME.to_string())
+                .clone(),
+        };
+
+        let virtio_rtc_device = Arc::new(Mutex::new(
+            virtio_devices::Rtc::new(
+                id.clone(),
+                self.force_access_platform | rtc_config.pci_common.iommu,
+                self.seccomp_action.clone(),
+                self.exit_evt
+                    .try_clone()
+                    .map_err(DeviceManagerError::EventFd)?,
+                state_from_id(snapshot, id.as_str())
+                    .map_err(DeviceManagerError::RestoreGetState)?,
+            )
+            .map_err(DeviceManagerError::CreateVirtioRtc)?,
+        ));
+        self.virtio_devices.push(MetaVirtioDevice {
+            virtio_device: Arc::clone(&virtio_rtc_device)
+                as Arc<Mutex<dyn virtio_devices::VirtioDevice>>,
+            pci_common: rtc_config.pci_common.clone(),
+            dma_handler: None,
+        });
+
+        // Fill the device tree with a new node. In case of restore, we
+        // know there is nothing to do, so we can simply override the
+        // existing entry.
+        self.device_tree
+            .lock()
+            .unwrap()
+            .insert(id.clone(), device_node!(id, virtio_rtc_device));
+
+        Ok(())
+    }
+
     fn make_generic_vhost_user_device(
         &mut self,
         generic_vhost_user_cfg: &mut GenericVhostUserConfig,