examples: Add vhost-user RNG support to chroot_vm

Adds --vhost-user-rng command line option to
specify a vhost-user RNG backend socket path.
When provided, the VM uses the external
vhost-user RNG device instead of the built-in
virtio-rng implementation.

Example usage:  ./examples/chroot_vm \
--vhost-user-rng=/tmp/vhost-rng.sock0 \
/ /bin/sh -c "head -c 32 /dev/hwrng | xxd"

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

Author: Dorinda Bassey

examples/chroot_vm.c

@@ -38,7 +38,8 @@ static void print_help(char *const name)
         "              --log=PATH            Write libkrun log to file or named pipe at PATH\n"
         "              --color-log=PATH      Write libkrun log to file or named pipe at PATH, use color\n"
         "              --net=NET_MODE        Set network mode\n"
-        "              --passt-socket=PATH   Instead of starting passt, connect to passt socket at PATH"
+        "              --passt-socket=PATH   Instead of starting passt, connect to passt socket at PATH\n"
+        "              --vhost-user-rng=PATH Use vhost-user RNG backend at socket PATH\n"
         "NET_MODE can be either TSI (default) or PASST\n"
         "\n"
         "NEWROOT:      the root directory of the vm\n"
@@ -48,12 +49,23 @@ static void print_help(char *const name)
     );
 }
 
+static bool check_krun_error(int err, const char *msg)
+{
+    if (err) {
+        errno = -err;
+        perror(msg);
+        return false;
+    }
+    return true;
+}
+
 static const struct option long_options[] = {
     { "help", no_argument, NULL, 'h' },
     { "log", required_argument, NULL, 'L' },
     { "color-log", required_argument, NULL, 'C' },
     { "net_mode", required_argument, NULL, 'N' },
     { "passt-socket", required_argument, NULL, 'P' },
+    { "vhost-user-rng", required_argument, NULL, 'V' },
     { NULL, 0, NULL, 0 }
 };
 
@@ -63,6 +75,7 @@ struct cmdline {
     uint32_t log_style;
     enum net_mode net_mode;
     char const *passt_socket_path;
+    char const *vhost_user_rng_socket;
     char const *new_root;
     char *const *guest_argv;
 };
@@ -89,6 +102,7 @@ bool parse_cmdline(int argc, char *const argv[], struct cmdline *cmdline)
         .show_help = false,
         .net_mode = NET_MODE_TSI,
         .passt_socket_path = NULL,
+        .vhost_user_rng_socket = NULL,
         .new_root = NULL,
         .guest_argv = NULL,
         .log_target = KRUN_LOG_TARGET_DEFAULT,
@@ -124,6 +138,9 @@ bool parse_cmdline(int argc, char *const argv[], struct cmdline *cmdline)
         case 'P':
             cmdline->passt_socket_path = optarg;
             break;
+        case 'V':
+            cmdline->vhost_user_rng_socket = optarg;
+            break;
         case '?':
             return false;
         default:
@@ -249,6 +266,19 @@ int main(int argc, char *const argv[])
         return -1;
     }
 
+    // Configure vhost-user RNG if requested
+    if (cmdline.vhost_user_rng_socket != NULL) {
+        // Test sentinel-terminated array: auto-detect queue count, use custom size
+        uint16_t custom_sizes[] = {512, 0};  // 0 = sentinel terminator
+
+        if (!check_krun_error(krun_add_vhost_user_device(ctx_id, KRUN_VIRTIO_DEVICE_RNG,
+                                                          cmdline.vhost_user_rng_socket, NULL, 0, custom_sizes),
+                              "Error adding vhost-user RNG device")) {
+            return -1;
+        }
+        printf("Using vhost-user RNG backend at %s (custom queue size: 512)\n", cmdline.vhost_user_rng_socket);
+    }
+
     // Raise RLIMIT_NOFILE to the maximum allowed to create some room for virtio-fs
     getrlimit(RLIMIT_NOFILE, &rlim);
     rlim.rlim_cur = rlim.rlim_max;