runtime-rs: support adding PCI bridges to qemu VM

At least one PCI bridge is necessary to hotplug PCI devices.  We only
support PCI (at this point at least) since that's what the go runtime
does (note that looking at the code in virtcontainers it might seem that
other bus types are supported, however when the bridge objects are passed
to govmm, all but PCI bridges are actually ignored).  The entire logic of
bridge setup is lifted from runtime-go for compatibility's sake.

Signed-off-by: Pavel Mores <pmores@redhat.com>

src/runtime-rs/crates/hypervisor/src/qemu/cmdline_generator.rs

@@ -1290,6 +1290,74 @@ impl ToQemuParams for DeviceIntelIommu {
     }
 }
 
+#[derive(Debug)]
+struct DevicePciBridge {
+    driver: String,
+    bus: String,
+    id: String,
+    chassis_nr: u32,
+    shpc: bool,
+    addr: u32,
+    io_reserve: String,
+    mem_reserve: String,
+    pref64_reserve: String,
+}
+
+impl DevicePciBridge {
+    fn new(config: &HypervisorConfig, bridge_idx: u32) -> DevicePciBridge {
+        DevicePciBridge {
+            // The go runtime doesn't support bridges other than PCI although
+            // PCIe should also be available.  Stick with the legacy behaviour
+            // of ignoring PCIe since it's not clear to me how to decide
+            // between the two.
+            driver: "pci-bridge".to_owned(),
+            bus: match config.machine_info.machine_type.as_str() {
+                "q35" | "virt" => "pcie.0",
+                _ => "pci.0",
+            }
+            .to_owned(),
+            id: format!("pci-bridge-{}", bridge_idx),
+            // Each bridge is required to be assigned a unique chassis id > 0.
+            chassis_nr: bridge_idx + 1,
+            shpc: false,
+            // 2 is documented by the go runtime as the first slot available
+            // for a bridge (on x86_64)
+            // (https://github.com/kata-containers/kata-containers/blob/99730256a2899c82d111400024621519d17ea15d/src/runtime/virtcontainers/qemu_arch_base.go#L212)
+            addr: 2 + bridge_idx,
+            // Values taken from the go runtime implementation which comments
+            // the choices as follows:
+            // Certain guest BIOS versions think !SHPC means no hotplug, and
+            // won't reserve the IO and memory windows that will be needed for
+            // devices added underneath this bridge.  This will only break for
+            // certain combinations of exact qemu, BIOS and guest kernel
+            // versions, but for consistency, just hint the usual default
+            // windows for a bridge (as the BIOS would use with SHPC) so that
+            // we can do ACPI hotplug.
+            // (https://github.com/kata-containers/kata-containers/blob/99730256a2899c82d111400024621519d17ea15d/src/runtime/virtcontainers/qemu.go#L2474)
+            io_reserve: "4k".to_owned(),
+            mem_reserve: "1m".to_owned(),
+            pref64_reserve: "1m".to_owned(),
+        }
+    }
+}
+
+#[async_trait]
+impl ToQemuParams for DevicePciBridge {
+    async fn qemu_params(&self) -> Result<Vec<String>> {
+        let mut params = Vec::new();
+        params.push(self.driver.clone());
+        params.push(format!("bus={}", self.bus));
+        params.push(format!("id={}", self.id));
+        params.push(format!("chassis_nr={}", self.chassis_nr));
+        params.push(format!("shpc={}", if self.shpc { "on" } else { "off" }));
+        params.push(format!("addr={}", self.addr));
+        params.push(format!("io-reserve={}", self.io_reserve));
+        params.push(format!("mem-reserve={}", self.mem_reserve));
+        params.push(format!("pref64-reserve={}", self.pref64_reserve));
+        Ok(vec!["-device".to_owned(), params.join(",")])
+    }
+}
+
 // Qemu provides methods and types for managing QEMU instances.
 // To manage a qemu instance after it has been launched you need
 // to pass the -qmp option during launch requesting the qemu instance
@@ -1484,6 +1552,11 @@ impl<'a> QemuCmdLine<'a> {
             qemu_cmd_line.add_rng();
         }
 
+        if qemu_cmd_line.bus_type() != VirtioBusType::Ccw && config.device_info.default_bridges > 0
+        {
+            qemu_cmd_line.add_bridges(config.device_info.default_bridges);
+        }
+
         Ok(qemu_cmd_line)
     }
 
@@ -1526,6 +1599,13 @@ impl<'a> QemuCmdLine<'a> {
         self.machine.set_kernel_irqchip("split");
     }
 
+    fn add_bridges(&mut self, count: u32) {
+        for idx in 0..count {
+            let bridge = DevicePciBridge::new(self.config, idx);
+            self.devices.push(Box::new(bridge));
+        }
+    }
+
     pub fn add_virtiofs_share(
         &mut self,
         virtiofsd_socket_path: &str,