qemu-nbd: add --discard option

Similar to --cache and --aio, this option mimics the discard suboption
of "-drive".

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

qemu-nbd.c

@@ -33,9 +33,10 @@
 #include <libgen.h>
 #include <pthread.h>
 
-#define SOCKET_PATH         "/var/lock/qemu-nbd-%s"
-#define QEMU_NBD_OPT_CACHE  1
-#define QEMU_NBD_OPT_AIO    2
+#define SOCKET_PATH          "/var/lock/qemu-nbd-%s"
+#define QEMU_NBD_OPT_CACHE   1
+#define QEMU_NBD_OPT_AIO     2
+#define QEMU_NBD_OPT_DISCARD 3
 
 static NBDExport *exp;
 static int verbose;
@@ -330,6 +331,7 @@ int main(int argc, char **argv)
 #ifdef CONFIG_LINUX_AIO
         { "aio", 1, NULL, QEMU_NBD_OPT_AIO },
 #endif
+        { "discard", 1, NULL, QEMU_NBD_OPT_DISCARD },
         { "shared", 1, NULL, 'e' },
         { "persistent", 0, NULL, 't' },
         { "verbose", 0, NULL, 'v' },
@@ -344,6 +346,7 @@ int main(int argc, char **argv)
     int ret;
     int fd;
     bool seen_cache = false;
+    bool seen_discard = false;
 #ifdef CONFIG_LINUX_AIO
     bool seen_aio = false;
 #endif
@@ -389,6 +392,15 @@ int main(int argc, char **argv)
             }
             break;
 #endif
+        case QEMU_NBD_OPT_DISCARD:
+            if (seen_discard) {
+                errx(EXIT_FAILURE, "--discard can only be specified once");
+            }
+            seen_discard = true;
+            if (bdrv_parse_discard_flags(optarg, &flags) == -1) {
+                errx(EXIT_FAILURE, "Invalid discard mode `%s'", optarg);
+            }
+            break;
         case 'b':
             bindto = optarg;
             break;

qemu-nbd.texi

@@ -35,6 +35,10 @@ Export QEMU disk image using NBD protocol.
 @item --aio=@var{aio}
   choose asynchronous I/O mode between @samp{threads} (the default)
   and @samp{native} (Linux only).
+@item --discard=@var{discard}
+  toggles whether @dfn{discard} (also known as @dfn{trim} or @dfn{unmap})
+  requests are ignored or passed to the filesystem.  The default is no
+  (@samp{--discard=ignore}).
 @item -c, --connect=@var{dev}
   connect @var{filename} to NBD device @var{dev}
 @item -d, --disconnect