[PATCH 030/104] virtiofsd: add --print-capabilities option

[Dr. David Alan Gilbert (git)]

From: Stefan Hajnoczi <address@hidden>

Add the --print-capabilities option as per vhost-user.rst "Backend
programs conventions".  Currently there are no advertised features.

Signed-off-by: Stefan Hajnoczi <address@hidden>
---
 docs/interop/vhost-user.json     |  4 +++-
 tools/virtiofsd/fuse_lowlevel.h  |  1 +
 tools/virtiofsd/helper.c         |  2 ++
 tools/virtiofsd/passthrough_ll.c | 12 ++++++++++++
 4 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/docs/interop/vhost-user.json b/docs/interop/vhost-user.json
index da6aaf51c8..d4ea1f7ac5 100644
--- a/docs/interop/vhost-user.json
+++ b/docs/interop/vhost-user.json
@@ -31,6 +31,7 @@
 # @rproc-serial: virtio remoteproc serial link
 # @scsi: virtio scsi
 # @vsock: virtio vsock transport
+# @fs: virtio fs (since 4.2)
 #
 # Since: 4.0
 ##
@@ -50,7 +51,8 @@
       'rpmsg',
       'rproc-serial',
       'scsi',
-      'vsock'
+      'vsock',
+      'fs'
   ]
 }
 
diff --git a/tools/virtiofsd/fuse_lowlevel.h b/tools/virtiofsd/fuse_lowlevel.h
index 081bb1abb3..6c63cb740c 100644
--- a/tools/virtiofsd/fuse_lowlevel.h
+++ b/tools/virtiofsd/fuse_lowlevel.h
@@ -1826,6 +1826,7 @@ struct fuse_cmdline_opts {
     int nodefault_subtype;
     int show_version;
     int show_help;
+    int print_capabilities;
     int clone_fd;
     unsigned int max_idle_threads;
 };
diff --git a/tools/virtiofsd/helper.c b/tools/virtiofsd/helper.c
index d4fff4fa53..4c9a3b2fc9 100644
--- a/tools/virtiofsd/helper.c
+++ b/tools/virtiofsd/helper.c
@@ -39,6 +39,7 @@ static const struct fuse_opt fuse_helper_opts[] = {
     FUSE_HELPER_OPT("--help", show_help),
     FUSE_HELPER_OPT("-V", show_version),
     FUSE_HELPER_OPT("--version", show_version),
+    FUSE_HELPER_OPT("--print-capabilities", print_capabilities),
     FUSE_HELPER_OPT("-d", debug),
     FUSE_HELPER_OPT("debug", debug),
     FUSE_HELPER_OPT("-d", foreground),
@@ -136,6 +137,7 @@ void fuse_cmdline_help(void)
 {
     printf("    -h   --help                print help\n"
            "    -V   --version             print version\n"
+           "    --print-capabilities       print vhost-user.json\n"
            "    -d   -o debug              enable debug output (implies -f)\n"
            "    -f                         foreground operation\n"
            "    --daemonize                run in background\n"
diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
index 23531d791d..68bacb6fc5 100644
--- a/tools/virtiofsd/passthrough_ll.c
+++ b/tools/virtiofsd/passthrough_ll.c
@@ -1298,6 +1298,14 @@ static struct fuse_lowlevel_ops lo_oper = {
     .lseek = lo_lseek,
 };
 
+/* Print vhost-user.json backend program capabilities */
+static void print_capabilities(void)
+{
+    printf("{\n");
+    printf("  \"type\": \"fs\"\n");
+    printf("}\n");
+}
+
 int main(int argc, char *argv[])
 {
     struct fuse_args args = FUSE_ARGS_INIT(argc, argv);
@@ -1328,6 +1336,10 @@ int main(int argc, char *argv[])
         fuse_lowlevel_version();
         ret = 0;
         goto err_out1;
+    } else if (opts.print_capabilities) {
+        print_capabilities();
+        ret = 0;
+        goto err_out1;
     }
 
     if (fuse_opt_parse(&args, &lo, lo_opts, NULL) == -1) {
-- 
2.23.0