vmm: DeviceConfig: Add fd field for an externally-opened vfio cdev

Add a new `fd: Option<i32>` field to DeviceConfig so a caller can
supply a pre-opened vfio cdev FD (e.g. /dev/vfio/devices/vfioN) in
addition to the existing sysfs path. The CLI `--device` option now
accepts `fd=<n>`, parsed alongside the existing options.

Signed-off-by: Bo Chen <bchen@crusoe.ai>
Assisted-by: Claude:Opus-4.7

vmm/src/config.rs

@@ -2413,7 +2413,7 @@ impl DebugConsoleConfig {
 
 impl DeviceConfig {
     pub const SYNTAX: &'static str = "Direct device assignment parameters \
-    \"path=<device_path>,iommu=on|off,id=<device_id>,\
+    \"path=<device_path>,fd=<vfio_cdev_fd>,iommu=on|off,id=<device_id>,\
     pci_segment=<segment_id>,pci_device_id=<pci_slot>,\
     x_nv_gpudirect_clique=<clique_id>,\
     x_exclude_mmap_bars=[<bar>...]\"";
@@ -2422,6 +2422,7 @@ impl DeviceConfig {
         let mut parser = OptionParser::new();
         parser
             .add("path")
+            .add("fd")
             .add_all(PciDeviceCommonConfig::OPTIONS_IOMMU)
             .add("x_nv_gpudirect_clique")
             .add("x_exclude_mmap_bars");
@@ -2432,6 +2433,7 @@ impl DeviceConfig {
             .get("path")
             .map(PathBuf::from)
             .ok_or(Error::ParseDevicePathMissing)?;
+        let fd = parser.convert::<i32>("fd").map_err(Error::ParseDevice)?;
         let x_nv_gpudirect_clique = parser
             .convert::<u8>("x_nv_gpudirect_clique")
             .map_err(Error::ParseDevice)?;
@@ -2443,6 +2445,7 @@ impl DeviceConfig {
         Ok(DeviceConfig {
             pci_common,
             path: Some(path),
+            fd,
             x_nv_gpudirect_clique,
             x_exclude_mmap_bars,
         })
@@ -4722,6 +4725,7 @@ id=\"{id}\",pci_segment={pci_segment},queue_sizes={queue_sizes}"
         DeviceConfig {
             pci_common: PciDeviceCommonConfig::default(),
             path: Some(PathBuf::from("/path/to/device")),
+            fd: None,
             x_nv_gpudirect_clique: None,
             x_exclude_mmap_bars: Vec::new(),
         }

vmm/src/vm_config.rs

@@ -760,12 +760,30 @@ pub struct DeviceConfig {
     pub pci_common: PciDeviceCommonConfig,
     #[serde(default)]
     pub path: Option<PathBuf>,
+    // FDs are not serialized and any deserialized value is invalid; see NetConfig::fds.
+    #[serde(default, deserialize_with = "deserialize_deviceconfig_fd")]
+    pub fd: Option<i32>,
     #[serde(default)]
     pub x_nv_gpudirect_clique: Option<u8>,
     #[serde(default)]
     pub x_exclude_mmap_bars: Vec<u64>,
 }
 
+fn deserialize_deviceconfig_fd<'de, D>(d: D) -> Result<Option<i32>, D::Error>
+where
+    D: serde::Deserializer<'de>,
+{
+    let invalid_fd: Option<i32> = Option::deserialize(d)?;
+    if invalid_fd.is_some() {
+        debug!(
+            "FD in 'DeviceConfig' won't be deserialized as it is most likely invalid now. Deserializing it as -1."
+        );
+        Ok(Some(-1))
+    } else {
+        Ok(None)
+    }
+}
+
 impl ApplyLandlock for DeviceConfig {
     fn apply_landlock(&self, landlock: &mut Landlock) -> LandlockResult<()> {
         let path = self