libxl: add option for discard support to xl disk configuration

Handle new boolean option discard/no-discard for disk configuration. It
is supposed to disable discard support if file based backing storage was
intentionally created non-sparse to avoid fragmentation of the file.

The option intended for the backend driver. A new boolean property
"discard-enable" is written to the backend node. An upcoming patch for
qemu will make use of this property. The kernel blkback driver may be
updated as well to disable discard for phy based backing storage.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>

docs/misc/xl-disk-configuration.txt

@@ -217,6 +217,23 @@ If in the future the bug is fixed properly this option will then be
 silently ignored.
 
 
+discard / no-discard
+---------------
+
+Description:           Request that backend advertise discard support to frontend
+Supported values:      discard
+                       no-discard
+Mandatory:             No
+Default value:         discard
+
+An advisory setting for the backend driver, specifying whether to
+advertise discard support (TRIM, UNMAP) to the frontend.  The real
+benefit of this option is to be able to force it off rather than on.  It
+can be used to disable "hole punching" for file based backends which
+were intentionally created non-sparse to avoid fragmentation of the
+file.
+
+
 ============================================
 DEPRECATED PARAMETERS, PREFIXES AND SYNTAXES
 ============================================

tools/libxl/check-xl-disk-parse

@@ -62,7 +62,8 @@ disk: {
     "removable": 0,
     "readwrite": 1,
     "is_cdrom": 0,
-    "direct_io_safe": false
+    "direct_io_safe": false,
+    "discard_enable": "True"
 }
 
 END
@@ -84,7 +85,8 @@ disk: {
     "removable": 1,
     "readwrite": 0,
     "is_cdrom": 1,
-    "direct_io_safe": false
+    "direct_io_safe": false,
+    "discard_enable": "False"
 }
 
 END
@@ -107,7 +109,8 @@ disk: {
     "removable": 0,
     "readwrite": 1,
     "is_cdrom": 0,
-    "direct_io_safe": false
+    "direct_io_safe": false,
+    "discard_enable": "True"
 }
 
 EOF
@@ -125,7 +128,8 @@ disk: {
     "removable": 1,
     "readwrite": 0,
     "is_cdrom": 1,
-    "direct_io_safe": false
+    "direct_io_safe": false,
+    "discard_enable": "False"
 }
 
 EOF
@@ -147,7 +151,8 @@ disk: {
     "removable": 1,
     "readwrite": 0,
     "is_cdrom": 1,
-    "direct_io_safe": false
+    "direct_io_safe": false,
+    "discard_enable": "False"
 }
 
 EOF
@@ -166,7 +171,8 @@ disk: {
     "removable": 0,
     "readwrite": 1,
     "is_cdrom": 0,
-    "direct_io_safe": false
+    "direct_io_safe": false,
+    "discard_enable": "True"
 }
 
 EOF
@@ -187,7 +193,8 @@ disk: {
     "removable": 0,
     "readwrite": 1,
     "is_cdrom": 0,
-    "direct_io_safe": false
+    "direct_io_safe": false,
+    "discard_enable": "True"
 }
 
 EOF
@@ -196,4 +203,63 @@ EOF
 # http://www.drbd.org/users-guide-emb/s-xen-configure-domu.html
 one 0 drbd:app01,hda,w
 
+expected <<END
+disk: {
+    "backend_domid": 0,
+    "backend_domname": null,
+    "pdev_path": "/some/disk/image.raw",
+    "vdev": "hda",
+    "backend": "unknown",
+    "format": "raw",
+    "script": null,
+    "removable": 0,
+    "readwrite": 1,
+    "is_cdrom": 0,
+    "direct_io_safe": false,
+    "discard_enable": "True"
+}
+
+END
+one 0  discard=on  vdev=hda target=/some/disk/image.raw
+one 0  discard=1   vdev=hda target=/some/disk/image.raw
+
+expected <<END
+disk: {
+    "backend_domid": 0,
+    "backend_domname": null,
+    "pdev_path": "/some/disk/image.raw",
+    "vdev": "hda",
+    "backend": "unknown",
+    "format": "raw",
+    "script": null,
+    "removable": 0,
+    "readwrite": 1,
+    "is_cdrom": 0,
+    "direct_io_safe": false,
+    "discard_enable": "False"
+}
+
+END
+one 0  discard vdev=hda target=/some/disk/image.raw
+one 0  discard vdev=hda target=/some/disk/image.raw
+
+expected <<END
+disk: {
+    "backend_domid": 0,
+    "backend_domname": null,
+    "pdev_path": "/some/disk/image.iso",
+    "vdev": "hda",
+    "backend": "unknown",
+    "format": "raw",
+    "script": null,
+    "removable": 1,
+    "readwrite": 0,
+    "is_cdrom": 1,
+    "direct_io_safe": false,
+    "discard_enable": "False"
+}
+
+END
+one 0  cdrom no-discard vdev=hda target=/some/disk/image.iso
+
 complete

tools/libxl/libxl.c

@@ -2209,6 +2209,9 @@ static void device_disk_add(libxl__egc *egc, uint32_t domid,
             flexarray_append(back, "direct-io-safe");
             flexarray_append(back, "1");
         }
+        flexarray_append_pair(back, "discard-enable",
+                              libxl_defbool_val(disk->discard_enable) ?
+                              "1" : "0");
 
         flexarray_append(front, "backend-id");
         flexarray_append(front, libxl__sprintf(gc, "%d", disk->backend_domid));

tools/libxl/libxl.h

@@ -101,6 +101,11 @@
  */
 #define LIBXL_HAVE_DEVICE_DISK_DIRECT_IO_SAFE 1
 
+/*
+ * The libxl_device_disk has the discard_enable field.
+ */
+#define LIBXL_HAVE_LIBXL_DEVICE_DISK_DISCARD_ENABLE 1
+
 /*
  * libxl ABI compatibility
  *

tools/libxl/libxl_types.idl

@@ -417,6 +417,7 @@ libxl_device_disk = Struct("device_disk", [
     ("readwrite", integer),
     ("is_cdrom", integer),
     ("direct_io_safe", bool),
+    ("discard_enable", libxl_defbool),
     ])
 
 libxl_device_nic = Struct("device_nic", [

tools/libxl/libxlu_disk.c

@@ -79,6 +79,7 @@ int xlu_disk_parse(XLU_Config *cfg,
         if (!disk->pdev_path || !strcmp(disk->pdev_path, ""))
             disk->format = LIBXL_DISK_FORMAT_EMPTY;
     }
+    libxl_defbool_setdefault(&disk->discard_enable, !!disk->readwrite);
 
     if (!disk->vdev) {
         xlu__disk_err(&dpc,0, "no vdev specified");

tools/libxl/libxlu_disk_l.l

@@ -174,6 +174,8 @@ backendtype=[^,]*,? { STRIP(','); setbackendtype(DPC,FROMEQUALS); }
 vdev=[^,]*,?	{ STRIP(','); SAVESTRING("vdev", vdev, FROMEQUALS); }
 script=[^,]*,?	{ STRIP(','); SAVESTRING("script", script, FROMEQUALS); }
 direct-io-safe,? { DPC->disk->direct_io_safe = 1; }
+discard,?	{ libxl_defbool_set(&DPC->disk->discard_enable, true); }
+no-discard,?	{ libxl_defbool_set(&DPC->disk->discard_enable, false); }
 
  /* the target magic parameter, eats the rest of the string */
 

xen/include/public/io/blkif.h

@@ -197,6 +197,16 @@
  *
  *------------------------- Backend Device Properties -------------------------
  *
+ * discard-enable
+ *      Values:         0/1 (boolean)
+ *      Default Value:  1
+ *
+ *      This optional property, set by the toolstack, instructs the backend
+ *      to offer discard to the frontend. If the property is missing the
+ *      backend should offer discard if the backing storage actually supports
+ *      it. This optional property, set by the toolstack, requests that the
+ *      backend offer, or not offer, discard to the frontend.
+ *
  * discard-alignment
  *      Values:         <uint32_t>
  *      Default Value:  0