Re: [PATCH for xen-4.4] libxl: add option for discard support to xl disk configuration

[Olaf Hering <olaf@aepfle.de>]

And just for reference, this is a version for our 4.4:

....

This change does not break ABI. Instead of adding a new member ->discard_enable
to struct libxl_device_disk the existing ->readwrite member is reused.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
---
 docs/misc/xl-disk-configuration.txt | 15 +++++++++++++++
 tools/libxl/libxl.c                 |  2 ++
 tools/libxl/libxl.h                 | 11 +++++++++++
 tools/libxl/libxlu_disk.c           |  3 +++
 tools/libxl/libxlu_disk_i.h         |  2 +-
 tools/libxl/libxlu_disk_l.l         |  4 ++++
 6 files changed, 36 insertions(+), 1 deletion(-)

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/libxl.c b/tools/libxl/libxl.c
index 2845ca4..9ed5062 100644
--- a/tools/libxl/libxl.c
+++ b/tools/libxl/libxl.c
@@ -2196,6 +2196,8 @@ 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 (disk->readwrite == LIBXL_HAVE_LIBXL_DEVICE_DISK_DISCARD_DISABLE_MAGIC)
+            flexarray_append_pair(back, "discard-enable", "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..021f7e4 100644
--- a/tools/libxl/libxl.h
+++ b/tools/libxl/libxl.h
@@ -95,6 +95,17 @@
 #define LIBXL_HAVE_BUILDINFO_EVENT_CHANNELS 1
 
 /*
+ * The libxl_device_disk lacks discard_enable field, disabling discard
+ * is supported without breaking the ABI. This is done by overloading
+ * struct libxl_device_disk->readwrite:
+ * readwrite == 0: disk is readonly, no discard
+ * readwrite == 1: disk is readwrite, backend driver may enable discard
+ * readwrite == MAGIC: disk is readwrite, backend driver should not offer
+ * discard to the frontend driver.
+ */
+#define LIBXL_HAVE_LIBXL_DEVICE_DISK_DISCARD_DISABLE_MAGIC 0xdcadU
+
+/*
  * libxl ABI compatibility
  *
  * The only guarantee which libxl makes regarding ABI compatibility
diff --git a/tools/libxl/libxlu_disk.c b/tools/libxl/libxlu_disk.c
index 18fe386..e596cb6 100644
--- a/tools/libxl/libxlu_disk.c
+++ b/tools/libxl/libxlu_disk.c
@@ -80,6 +80,9 @@ int xlu_disk_parse(XLU_Config *cfg,
             disk->format = LIBXL_DISK_FORMAT_EMPTY;
     }
 
+    if (disk->readwrite && dpc.disable_discard)
+        disk->readwrite = LIBXL_HAVE_LIBXL_DEVICE_DISK_DISCARD_DISABLE_MAGIC;
+
     if (!disk->vdev) {
         xlu__disk_err(&dpc,0, "no vdev specified");
         goto x_err;
diff --git a/tools/libxl/libxlu_disk_i.h b/tools/libxl/libxlu_disk_i.h
index 4fccd4a..9db3002 100644
--- a/tools/libxl/libxlu_disk_i.h
+++ b/tools/libxl/libxlu_disk_i.h
@@ -10,7 +10,7 @@ typedef struct {
     void *scanner;
     YY_BUFFER_STATE buf;
     libxl_device_disk *disk;
-    int access_set, had_depr_prefix;
+    int access_set, disable_discard, had_depr_prefix;
     const char *spec;
 } DiskParseContext;
 
diff --git a/tools/libxl/libxlu_disk_l.l b/tools/libxl/libxlu_disk_l.l
index 7c4e7f1..ecc30ae 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,?	{ DPC->disable_discard = 0; }
+discard=1,?	{ DPC->disable_discard = 0; }
+discard=off,?	{ DPC->disable_discard = 1; }
+discard=0,?	{ DPC->disable_discard = 1; }
 
  /* the target magic parameter, eats the rest of the string */