vmm: Accept an externally-opened iommufd FD

The CLI `--platform` option now accepts `iommufd_fd=<n>` alongside the
existing `iommufd=on|off`.

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

vmm/src/vm_config.rs

@@ -159,6 +159,9 @@ pub struct PlatformConfig {
     pub sev_snp: bool,
     #[serde(default)]
     pub iommufd: bool,
+    // FDs are not serialized and any deserialized value is invalid; see NetConfig::fds.
+    #[serde(default, deserialize_with = "deserialize_platformconfig_iommufd_fd")]
+    pub iommufd_fd: Option<i32>,
     #[serde(default = "default_platformconfig_vfio_p2p_dma")]
     pub vfio_p2p_dma: bool,
 }
@@ -207,6 +210,21 @@ impl PlatformConfig {
     }
 }
 
+fn deserialize_platformconfig_iommufd_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 'PlatformConfig::iommufd_fd' won't be deserialized as it is most likely invalid now. Deserializing it as -1."
+        );
+        Ok(Some(-1))
+    } else {
+        Ok(None)
+    }
+}
+
 pub const DEFAULT_PCI_SEGMENT_APERTURE_WEIGHT: u32 = 1;
 
 fn default_pci_segment_aperture_weight() -> u32 {