fix(qemu): select the virt machine for riscv64

qemu-system-riscv64's default machine is spike, not virt: spike has no
PCI bus, so virtio-net-pci failed with "No 'PCI' bus found", and no
16550 UART, so console=ttyS0 output went nowhere. Pass -machine virt
explicitly for riscv64; other architectures keep their emulator's
default.

crates/ecr/src/qemu_vm.rs

@@ -125,10 +125,16 @@ pub fn launch_qemu(config: QemuConfig) -> Result<()> {
     veprintln!("  Kernel append: {}", kernel_append);
 
     // Build QEMU arguments
+    // -machine virt is selected explicitly on riscv64 (see machine_for_arch)
     // -display none suppresses VGA/BIOS output
     // -serial mon:stdio connects serial console to terminal with QEMU monitor muxed
     // -no-reboot makes QEMU exit when the guest requests poweroff/reboot
-    let mut args = vec![
+    let mut args: Vec<String> = Vec::new();
+    if let Some(machine) = machine_for_arch(&config.arch) {
+        args.push("-machine".to_string());
+        args.push(machine.to_string());
+    }
+    args.extend(vec![
         "-kernel".to_string(),
         config.kernel_path.to_string_lossy().to_string(),
         "-initrd".to_string(),
@@ -146,7 +152,7 @@ pub fn launch_qemu(config: QemuConfig) -> Result<()> {
         "user,id=net0".to_string(),
         "-device".to_string(),
         "virtio-net-pci,netdev=net0".to_string(),
-    ];
+    ]);
 
     // Add KVM acceleration if available
     if use_kvm {
@@ -182,6 +188,19 @@ fn qemu_binary_for_arch(arch: &str) -> String {
     format!("qemu-system-{}", arch_enum.qemu_system_name())
 }
 
+/// Get the QEMU machine to select for the target architecture, if any.
+///
+/// qemu-system-riscv64's default machine is `spike`, which has neither a
+/// PCI bus (virtio-net-pci fails with "No 'PCI' bus found") nor a 16550
+/// UART (console=ttyS0 output goes nowhere); the `virt` board has both,
+/// plus bundled OpenSBI firmware for -kernel boot.
+fn machine_for_arch(arch: &str) -> Option<&'static str> {
+    match crate::utils::Arch::from_str(arch) {
+        crate::utils::Arch::Riscv64 => Some("virt"),
+        _ => None,
+    }
+}
+
 /// Get architecture suffix for package names
 fn get_arch_package_suffix(arch: &str) -> &'static str {
     crate::utils::Arch::from_str(arch).qemu_package_suffix()
@@ -601,6 +620,21 @@ mod tests {
     use super::*;
     use std::io::Read as _;
 
+    #[test]
+    fn test_machine_for_arch() {
+        // riscv64 must select the virt machine explicitly: the
+        // qemu-system-riscv64 default is `spike`, which has no PCI bus and
+        // no 16550 UART
+        assert_eq!(machine_for_arch("riscv64"), Some("virt"));
+        // Other architectures keep their emulator's default machine
+        assert_eq!(machine_for_arch("amd64"), None);
+        assert_eq!(machine_for_arch("x86_64"), None);
+        assert_eq!(machine_for_arch("arm64"), None);
+        assert_eq!(machine_for_arch("aarch64"), None);
+        assert_eq!(machine_for_arch("ppc64le"), None);
+        assert_eq!(machine_for_arch("s390x"), None);
+    }
+
     /// Parse a newc cpio archive into (name, ino, mode, nlink, file_size, data) tuples
     fn parse_cpio(archive: Vec<u8>) -> Vec<(String, u32, u32, u32, u32, Vec<u8>)> {
         let mut cursor = std::io::Cursor::new(archive);