libkrun: Add API constants and example for vhost-user console

Add public API constants for vhost-user console devices and example
usage in chroot_vm. The underlying support already exists via the
generic VhostUserDevice wrapper.

Console devices require 4 queues for multiport support (receiveq,
transmitq, control receiveq, control transmitq).

Example integration:
- Added --vhost-user-console option to chroot_vm
- Available as /dev/hvc1 in the guest (implicit console uses hvc0)
- Can be used with vhost-device-console for remote console access

Example workflow:
  Terminal 1: Start vhost-device-console backend
  vhost-device-console --socket-path=/tmp/console.sock \
  --socket-count=1 --tcp-port=12346 --backend=network

  Terminal 2: Connect to console (before starting VM)
    nc localhost 12346

  Terminal 3: Start VM with vhost-user console
  chroot_vm --vhost-user-console=/tmp/console.sock0 / /bin/sh
  In guest: Test with echo "hello" > /dev/hvc1

Signed-off-by: Dorinda Bassey <dbassey@redhat.com>

Author: Dorinda Bassey

examples/chroot_vm.c

@@ -43,6 +43,7 @@ static void print_help(char *const name)
         "              --vhost-user-snd=PATH Use vhost-user sound backend at socket PATH\n"
         "              --vhost-user-vsock=PATH Use vhost-user vsock backend at socket PATH\n"
         "              --vhost-user-can=PATH Use vhost-user CAN backend at socket PATH\n"
+        "              --vhost-user-console=PATH Use vhost-user console backend at socket PATH\n"
         "NET_MODE can be either TSI (default) or PASST\n"
         "\n"
         "NEWROOT:      the root directory of the vm\n"
@@ -72,6 +73,7 @@ static const struct option long_options[] = {
     { "vhost-user-snd", required_argument, NULL, 'S' },
     { "vhost-user-vsock", required_argument, NULL, 'K' },
     { "vhost-user-can", required_argument, NULL, 'A' },
+    { "vhost-user-console", required_argument, NULL, 'O' },
     { NULL, 0, NULL, 0 }
 };
 
@@ -85,6 +87,7 @@ struct cmdline {
     char const *vhost_user_snd_socket;
     char const *vhost_user_vsock_socket;
     char const *vhost_user_can_socket;
+    char const *vhost_user_console_socket;
     char const *new_root;
     char *const *guest_argv;
 };
@@ -115,6 +118,7 @@ bool parse_cmdline(int argc, char *const argv[], struct cmdline *cmdline)
         .vhost_user_snd_socket = NULL,
         .vhost_user_vsock_socket = NULL,
         .vhost_user_can_socket = NULL,
+        .vhost_user_console_socket = NULL,
         .new_root = NULL,
         .guest_argv = NULL,
         .log_target = KRUN_LOG_TARGET_DEFAULT,
@@ -162,6 +166,9 @@ bool parse_cmdline(int argc, char *const argv[], struct cmdline *cmdline)
         case 'A':
             cmdline->vhost_user_can_socket = optarg;
             break;
+        case 'O':
+            cmdline->vhost_user_console_socket = optarg;
+            break;
         case '?':
             return false;
         default:
@@ -342,6 +349,19 @@ int main(int argc, char *const argv[])
         printf("Using vhost-user CAN backend at %s\n", cmdline.vhost_user_can_socket);
     }
 
+    // Configure vhost-user console if requested
+    if (cmdline.vhost_user_console_socket != NULL) {
+        if (!check_krun_error(krun_add_vhost_user_device(ctx_id, KRUN_VIRTIO_DEVICE_CONSOLE,
+                                                          cmdline.vhost_user_console_socket, NULL,
+                                                          KRUN_VHOST_USER_CONSOLE_NUM_QUEUES,
+                                                          KRUN_VHOST_USER_CONSOLE_QUEUE_SIZES),
+                              "Error adding vhost-user console device")) {
+            return -1;
+        }
+        printf("Using vhost-user console backend at %s (available as /dev/hvc1 in guest)\n", cmdline.vhost_user_console_socket);
+        printf("Test with: echo 'hello' > /dev/hvc1\n");
+    }
+
     // Raise RLIMIT_NOFILE to the maximum allowed to create some room for virtio-fs
     getrlimit(RLIMIT_NOFILE, &rlim);
     rlim.rlim_cur = rlim.rlim_max;

include/libkrun.h

@@ -751,11 +751,20 @@ int32_t krun_set_snd_device(uint32_t ctx_id, bool enable);
  * Vhost-user device types.
  * These correspond to virtio device type IDs for devices.
  */
+#define KRUN_VIRTIO_DEVICE_CONSOLE 3
 #define KRUN_VIRTIO_DEVICE_RNG 4
 #define KRUN_VIRTIO_DEVICE_VSOCK 19
 #define KRUN_VIRTIO_DEVICE_SND 25
 #define KRUN_VIRTIO_DEVICE_CAN 36
 
+/**
+ * Vhost-user console device default queue configuration.
+ * Console device uses 4 queues for multiport support:
+ * receiveq (idx 0), transmitq (idx 1), control receiveq (idx 2), control transmitq (idx 3).
+ */
+#define KRUN_VHOST_USER_CONSOLE_NUM_QUEUES 4
+#define KRUN_VHOST_USER_CONSOLE_QUEUE_SIZES ((uint16_t[]){128, 128, 64, 64})
+
 /**
  * Vhost-user RNG device default queue configuration.
  * Use these when you want explicit defaults instead of auto-detection.