Have tapback announce the discard capability for vbds

The discard capability is announced for vbds if the following two conditions
apply:
* The backing tapdisk indicates discard support for the image that is
  attached to the vbd (i.e. block-aio is used in combination with a capable
  filesystem)
* tapback is not run with the --nodiscard overwrite

If discard is enabled, the discard granularity (discard_granularity) is
statically announced as the sector size of the vbd. The alignment
(discard_alignment) is statically announced as 0.
Secure discard (discard_secure) is never guaranteed.

This commit has been dev-tested using:
* v8 Windows PV drivers that include XenDisk and thereby implement discard
* Linux xen-blkfront that implements discard

Signed-off-by: Robert Breker <robert.breker@citrix.com>

control/tap-ctl-info.c

@@ -25,7 +25,8 @@
 #include "tap-ctl.h"
 
 int tap_ctl_info(pid_t pid, unsigned long long *sectors,
-		unsigned int *sector_size, unsigned int *info, const int minor)
+		unsigned int *sector_size, unsigned int *info,
+		bool * discard_supported, const int minor)
 {
     tapdisk_message_t message;
     int err;
@@ -49,6 +50,7 @@ int tap_ctl_info(pid_t pid, unsigned long long *sectors,
         *sectors = message.u.image.sectors;
         *sector_size = message.u.image.sector_size;
         *info = message.u.image.info;
+        *discard_supported = message.u.image.discard_supported;
         return 0;
     } else if (TAPDISK_MESSAGE_ERROR == message.type) {
        return -message.u.response.error;

drivers/tapdisk-control.c

@@ -1193,6 +1193,7 @@ tapdisk_control_disk_info(
         image->sectors = vbd->disk_info.size;
         image->sector_size = vbd->disk_info.sector_size;
         image->info = vbd->disk_info.info;
+        image->discard_supported = vbd->disk_info.discard_supported;
     }
     return err;
 }

include/tap-ctl.h

@@ -176,7 +176,8 @@ int tap_ctl_disconnect_xenblkif(const pid_t pid, const domid_t domid,
  *
  */
 int tap_ctl_info(pid_t pid, unsigned long long *sectors, unsigned int
-		*sector_size, unsigned int *info, const int minor);
+		*sector_size, unsigned int *info, bool *discard_supported,
+		const int minor);
 
 /**
  * Parses a type:/path/to/file string, storing the type and path to the output

include/tapdisk-message.h

@@ -19,6 +19,7 @@
 #define _TAPDISK_MESSAGE_H_
 
 #include <inttypes.h>
+#include <stdbool.h>
 #include <sys/types.h>
 
 /*
@@ -70,6 +71,7 @@ struct tapdisk_message_image {
 	uint64_t                         sectors;
 	uint32_t                         sector_size;
 	uint32_t                         info;
+	bool                             discard_supported;
 };
 
 struct tapdisk_message_string {

tapback/backend.c

@@ -344,7 +344,7 @@ physical_device_changed(vbd_t *device) {
      * get the VBD parameters from the tapdisk
      */
     if ((err = tap_ctl_info(device->tap->pid, &device->sectors,
-                    &device->sector_size, &info,
+                    &device->sector_size, &info, &device->discard_supported,
                     device->minor))) {
         WARN(device, "error retrieving disk characteristics: %s\n",
                 strerror(-err));

tapback/frontend.c

@@ -259,6 +259,10 @@ connect_frontend(vbd_t *device) {
     int err = 0;
     xs_transaction_t xst = XBT_NULL;
     bool abort_transaction = false;
+    unsigned int discard_granularity = 0;
+    unsigned int discard_alignment = 0;
+    int discard_secure = 0;
+    int feature_discard = 0;
 
     ASSERT(device);
 
@@ -272,9 +276,47 @@ connect_frontend(vbd_t *device) {
         abort_transaction = true;
 
         /*
-         * FIXME blkback writes discard-granularity, discard-alignment,
-         * discard-secure, feature-discard but we don't.
+         * Prepare for discard
          */
+        if (device->discard_supported == true && /* backing driver supports */
+            device->backend->discard == true  && /* discard enabled */
+            device->mode == true && /* writable vbd */
+            device->cdrom == false) { /* not a CD */
+            INFO(device, "Discard enabled.");
+            discard_granularity = device->sector_size;
+            discard_alignment = 0;
+            discard_secure = 0;
+            feature_discard = 1;
+        } else
+            INFO(device, "Discard disabled.");
+        if ((err = tapback_device_printf(device, xst, "discard-granularity",
+                                         true, "%u", discard_granularity ))) {
+            WARN(device, "failed to write discard_granularity: %s\n",
+                                        strerror(-err));
+            break;
+        }
+
+        if ((err = tapback_device_printf(device, xst, "discard-alignment",
+                                         true, "%u", discard_alignment ))) {
+            WARN(device, "failed to write discard_alignment: %s\n",
+                                        strerror(-err));
+            break;
+        }
+
+        if ((err = tapback_device_printf(device, xst, "discard-secure",
+                                         true, "%d", discard_secure ))) {
+            WARN(device, "failed to write discard-secure: %s\n",
+                                        strerror(-err));
+            break;
+        }
+
+        if ((err = tapback_device_printf(device, xst, "feature-discard",
+                                         true, "%d", feature_discard ))) {
+            WARN(device, "failed to write feature_discard: %s\n",
+                                        strerror(-err));
+            break;
+        }
+
 
         /*
 		 * Write the number of sectors, sector size, info, and barrier support

tapback/tapback.c

@@ -219,7 +219,8 @@ tapback_write_pid(const char *pidfile)
  */
 static inline backend_t *
 tapback_backend_create(const char *name, const char *pidfile,
-        const domid_t domid, const bool barrier)
+                       const domid_t domid, const bool barrier,
+                       const bool discard)
 {
     int err;
     int len;
@@ -254,7 +255,8 @@ tapback_backend_create(const char *name, const char *pidfile,
         goto out;
     }
 
-	backend->barrier = barrier;
+    backend->barrier = barrier;
+    backend->discard = discard;
 
     backend->path = NULL;
 
@@ -501,7 +503,8 @@ usage(FILE * const stream, const char * const prog)
             "\t[-d|--debug]\n"
 			"\t[-h|--help]\n"
             "\t[-v|--verbose]\n"
-			"\t[-b]--nobarrier]\n"
+            "\t[-b]--nobarrier]\n"
+            "\t[-s]--nodiscard]\n"
             "\t[-n|--name]\n", prog);
 }
 
@@ -567,7 +570,8 @@ int main(int argc, char **argv)
     int err = 0;
 	backend_t *backend = NULL;
     domid_t opt_domid = 0;
-	bool opt_barrier = true;
+    bool opt_barrier = true;
+    bool opt_discard = true;
 
 	if (access("/dev/xen/gntdev", F_OK ) == -1) {
 		WARN(NULL, "grant device does not exist\n");
@@ -594,12 +598,13 @@ int main(int argc, char **argv)
             {"name", 0, NULL, 'n'},
             {"pidfile", 0, NULL, 'p'},
             {"domain", 0, NULL, 'x'},
-			{"nobarrier", 0, NULL, 'b'},
+            {"nobarrier", 0, NULL, 'b'},
+            {"nodiscard", 0, NULL, 's'},
 
         };
         int c;
 
-        c = getopt_long(argc, argv, "hdvn:p:x:b", longopts, NULL);
+        c = getopt_long(argc, argv, "hdvn:p:x:b:s", longopts, NULL);
         if (c < 0)
             break;
 
@@ -636,9 +641,14 @@ int main(int argc, char **argv)
             }
             INFO(NULL, "only serving domain %d\n", opt_domid);
             break;
-		case 'b':
-			opt_barrier = false;
-			break;
+        case 'b':
+            /* nobarrier */
+	    opt_barrier = false;
+	    break;
+        case 's':
+            /* nodiscard */
+            opt_discard = false;
+            break;
         case '?':
             goto usage;
         }
@@ -673,7 +683,7 @@ int main(int argc, char **argv)
     }
 
 	backend = tapback_backend_create(opt_name, opt_pidfile, opt_domid,
-			opt_barrier);
+                                         opt_barrier, opt_discard);
 	if (!backend) {
 		err = errno;
         WARN(NULL, "error creating back-end: %s\n", strerror(err));

tapback/tapback.h

@@ -195,6 +195,12 @@ typedef struct backend {
 	 * Tells whether we support write I/O barriers.
 	 */
 	bool barrier;
+
+	/**
+         * Whether discard is supposed to be enabled if supported by the
+         * tapdisk.
+         */
+        bool discard;
 } backend_t;
 
 /**
@@ -271,6 +277,11 @@ typedef struct vbd {
      */
     unsigned long long sectors;
 
+    /**
+     * Whether the backing driver supports discard, supplied by the tapdisk.
+     */
+    bool discard_supported;
+
     /**
      * VDISK_???, defined in include/xen/interface/io/blkif.h.
      */