agent: fix OVMF nested PCI path resolution

Treat three-segment PCI paths as pxb-only when the bus indicates pxb,
preventing wrong root-bus lookup for nested OVMF hot-plug devices.

Signed-off-by: Fabiano Fidêncio <ffidencio@nvidia.com>
Assisted-by: OpenAI Codex <codex@openai.com>

src/agent/src/linux_abi.rs

@@ -26,15 +26,29 @@ pub fn create_pci_root_bus_path(root_complex: &str) -> String {
     format!("/devices/pci0000:{root_complex}")
 }
 
+/// Minimum guest PCI bus number used by pxb-pcie NUMA expander bridges in the
+/// runtime (see `busNrSpacing` in createNUMAPCIeTopology).
+const PXB_PCIE_ROOT_BUS_MIN: u8 = 0x20;
+
+fn is_pxb_pcie_root_complex(segment: &str) -> bool {
+    u8::from_str_radix(segment, 16)
+        .map(|bus| bus >= PXB_PCIE_ROOT_BUS_MIN)
+        .unwrap_or(false)
+}
+
 // Parses a device tree path into a (root_complex, PCI path) pair.
 //
-// Supports two formats:
-//   - Full NUMA path: "root_complex/bus/device" (e.g. "10/00/02") where the
-//     first segment is the root complex and the rest form the PCI path.
-//   - Legacy path: "bus/device" (e.g. "00/02") which defaults to root complex "00".
+// Supports three formats:
+//   - NUMA pxb-pcie path: "root_complex/bus/device" (e.g. "20/00/02") where
+//     the first segment is a pxb-pcie guest bus number (>= 0x20) and the rest
+//     form the PCI path under that root complex.
+//   - Nested hot-plug path on pci0000:00: "parent/bridge/device" (e.g.
+//     "02/00/01") used by OVMF Q35 networking; stays on root complex "00".
+//   - Legacy path: "bus/device" (e.g. "00/02") which defaults to root complex
+//     "00".
 pub fn pcipath_from_dev_tree_path(dev_tree_path: &str) -> Result<(&str, pci::Path)> {
     let segments: Vec<&str> = dev_tree_path.split('/').collect();
-    if segments.len() >= 3 {
+    if segments.len() >= 3 && is_pxb_pcie_root_complex(segments[0]) {
         let root_complex = segments[0];
         let pci_part = &dev_tree_path[root_complex.len() + 1..];
         let pci_path = pci::Path::from_str(pci_part).with_context(|| {
@@ -141,3 +155,36 @@ pub const U_EVENT_SUB_SYSTEM: &str = "SUBSYSTEM";
 pub const U_EVENT_SEQ_NUM: &str = "SEQNUM";
 pub const U_EVENT_DEV_NAME: &str = "DEVNAME";
 pub const U_EVENT_INTERFACE: &str = "INTERFACE";
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_pcipath_from_dev_tree_path_legacy() {
+        let (root, path) = pcipath_from_dev_tree_path("02/01").unwrap();
+        assert_eq!(root, "00");
+        assert_eq!(path.len(), 2);
+        assert_eq!(path[0].slot(), 0x02);
+        assert_eq!(path[1].slot(), 0x01);
+    }
+
+    #[test]
+    fn test_pcipath_from_dev_tree_path_nested_ovmf() {
+        let (root, path) = pcipath_from_dev_tree_path("02/00/01").unwrap();
+        assert_eq!(root, "00");
+        assert_eq!(path.len(), 3);
+        assert_eq!(path[0].slot(), 0x02);
+        assert_eq!(path[1].slot(), 0x00);
+        assert_eq!(path[2].slot(), 0x01);
+    }
+
+    #[test]
+    fn test_pcipath_from_dev_tree_path_numa_pxb() {
+        let (root, path) = pcipath_from_dev_tree_path("20/00/02").unwrap();
+        assert_eq!(root, "20");
+        assert_eq!(path.len(), 2);
+        assert_eq!(path[0].slot(), 0x00);
+        assert_eq!(path[1].slot(), 0x02);
+    }
+}