[Xen-devel] [PATCH v2] libxl: add option for discard support to xl disk configuration

[Olaf Hering <olaf@xxxxxxxxx>]

Handle new option discard=on|off 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 is a boolean and 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.

v2:
rename xenstore property from discard_enable to discard-enable
update description in xl-disk-configuration.txt
use libxl_defbool as type for discard_enable
update check-xl-disk-parse to use "<default>"
add LIBXL_HAVE_LIBXL_DEVICE_DISK_DISCARD_ENABLE to libxl.h

Signed-off-by: Olaf Hering <olaf@xxxxxxxxx>
---
 docs/misc/xl-disk-configuration.txt | 15 +++++++++++++++
 tools/libxl/check-xl-disk-parse     | 21 ++++++++++++++-------
 tools/libxl/libxl.c                 |  3 +++
 tools/libxl/libxl.h                 |  5 +++++
 tools/libxl/libxl_types.idl         |  1 +
 tools/libxl/libxlu_disk_l.l         |  4 ++++
 xen/include/public/io/blkif.h       |  8 ++++++++
 7 files changed, 50 insertions(+), 7 deletions(-)

diff --git a/docs/misc/xl-disk-configuration.txt 
b/docs/misc/xl-disk-configuration.txt
index 5bd456d..c9fd9bd 100644
--- a/docs/misc/xl-disk-configuration.txt
+++ b/docs/misc/xl-disk-configuration.txt
@@ -178,6 +178,21 @@ information to be interpreted by the executable program 
<script>,
 These scripts are normally called "block-<script>".
 
 
+discard=<boolean>
+---------------
+
+Description:           Instruct backend to advertise discard support to 
frontend
+Supported values:      on, off, 0, 1
+Mandatory:             No
+Default value:         on if, available for that backend typ
+
+This option is an advisory setting for the backend driver, depending of the
+value, 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 allows
+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
diff --git a/tools/libxl/check-xl-disk-parse b/tools/libxl/check-xl-disk-parse
index 797277c..4d26ca3 100755
--- a/tools/libxl/check-xl-disk-parse
+++ b/tools/libxl/check-xl-disk-parse
@@ -61,7 +61,8 @@ disk: {
     "script": null,
     "removable": 0,
     "readwrite": 1,
-    "is_cdrom": 0
+    "is_cdrom": 0,
+    "discard_enable": "<default>"
 }
 
 END
@@ -82,7 +83,8 @@ disk: {
     "script": null,
     "removable": 1,
     "readwrite": 0,
-    "is_cdrom": 1
+    "is_cdrom": 1,
+    "discard_enable": "<default>"
 }
 
 END
@@ -104,7 +106,8 @@ disk: {
     "script": null,
     "removable": 0,
     "readwrite": 1,
-    "is_cdrom": 0
+    "is_cdrom": 0,
+    "discard_enable": "<default>"
 }
 
 EOF
@@ -121,7 +124,8 @@ disk: {
     "script": null,
     "removable": 1,
     "readwrite": 0,
-    "is_cdrom": 1
+    "is_cdrom": 1,
+    "discard_enable": "<default>"
 }
 
 EOF
@@ -142,7 +146,8 @@ disk: {
     "script": null,
     "removable": 1,
     "readwrite": 0,
-    "is_cdrom": 1
+    "is_cdrom": 1,
+    "discard_enable": "<default>"
 }
 
 EOF
@@ -160,7 +165,8 @@ disk: {
     "script": "block-iscsi",
     "removable": 0,
     "readwrite": 1,
-    "is_cdrom": 0
+    "is_cdrom": 0,
+    "discard_enable": "<default>"
 }
 
 EOF
@@ -180,7 +186,8 @@ disk: {
     "script": "block-drbd",
     "removable": 0,
     "readwrite": 1,
-    "is_cdrom": 0
+    "is_cdrom": 0,
+    "discard_enable": "<default>"
 }
 
 EOF
diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
index 2845ca4..64d081b 100644
--- a/tools/libxl/libxl.c
+++ b/tools/libxl/libxl.c
@@ -2196,6 +2196,9 @@ static void device_disk_add(libxl__egc *egc, uint32_t 
domid,
         flexarray_append(back, disk->readwrite ? "w" : "r");
         flexarray_append(back, "device-type");
         flexarray_append(back, disk->is_cdrom ? "cdrom" : "disk");
+        if (!libxl_defbool_is_default(disk->discard_enable))
+            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));
diff --git a/tools/libxl/libxl.h b/tools/libxl/libxl.h
index 12d6c31..f26ef3c 100644
--- a/tools/libxl/libxl.h
+++ b/tools/libxl/libxl.h
@@ -95,6 +95,11 @@
 #define LIBXL_HAVE_BUILDINFO_EVENT_CHANNELS 1
 
 /*
+ * The libxl_device_disk has the discard_enable field.
+ */
+#define LIBXL_HAVE_LIBXL_DEVICE_DISK_DISCARD_ENABLE 1
+
+/*
  * libxl ABI compatibility
  *
  * The only guarantee which libxl makes regarding ABI compatibility
diff --git a/tools/libxl/libxl_types.idl b/tools/libxl/libxl_types.idl
index 649ce50..6575515 100644
--- a/tools/libxl/libxl_types.idl
+++ b/tools/libxl/libxl_types.idl
@@ -415,6 +415,7 @@ libxl_device_disk = Struct("device_disk", [
     ("removable", integer),
     ("readwrite", integer),
     ("is_cdrom", integer),
+    ("discard_enable", libxl_defbool),
     ])
 
 libxl_device_nic = Struct("device_nic", [
diff --git a/tools/libxl/libxlu_disk_l.l b/tools/libxl/libxlu_disk_l.l
index 7c4e7f1..2585bee 100644
--- a/tools/libxl/libxlu_disk_l.l
+++ b/tools/libxl/libxlu_disk_l.l
@@ -173,6 +173,10 @@ backendtype=[^,]*,? { STRIP(','); 
setbackendtype(DPC,FROMEQUALS); }
 
 vdev=[^,]*,?   { STRIP(','); SAVESTRING("vdev", vdev, FROMEQUALS); }
 script=[^,]*,? { STRIP(','); SAVESTRING("script", script, FROMEQUALS); }
+discard=on,?   { libxl_defbool_set(&DPC->disk->discard_enable, true); }
+discard=1,?    { libxl_defbool_set(&DPC->disk->discard_enable, true); }
+discard=off,?  { libxl_defbool_set(&DPC->disk->discard_enable, false); }
+discard=0,?    { libxl_defbool_set(&DPC->disk->discard_enable, false); }
 
  /* the target magic parameter, eats the rest of the string */
 
diff --git a/xen/include/public/io/blkif.h b/xen/include/public/io/blkif.h
index 542f123..4704fe6 100644
--- a/xen/include/public/io/blkif.h
+++ b/xen/include/public/io/blkif.h
@@ -175,6 +175,14 @@
  *
  *------------------------- 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.
+ *
  * discard-alignment
  *      Values:         <uint32_t>
  *      Default Value:  0

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