[libvirt] [PATCH] qemu: Advertise <qcow2discard/> in capabilities if discard works for qcow2 backing files.

[Richard W.M. Jones]

Qemu since 1.5 supports discard.  However until qemu 1.6 it didn't
work for qcow2 files.  Ref:

http://wiki.qemu.org/ChangeLog/1.5
http://wiki.qemu.org/ChangeLog/1.6

If qemu >= 1.6, add this to guest capabilities:

    <features>
      <drivediscard/>
      <qcow2discard/>   <---
    </features>
---
 src/conf/capabilities.c                       |  3 ++-
 src/qemu/qemu_capabilities.c                  | 13 +++++++++++++
 src/qemu/qemu_capabilities.h                  |  1 +
 tests/qemucapabilitiesdata/caps_1.6.0-1.caps  |  1 +
 tests/qemucapabilitiesdata/caps_1.6.50-1.caps |  1 +
 5 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/src/conf/capabilities.c b/src/conf/capabilities.c
index ff5912a..08c9b73 100644
--- a/src/conf/capabilities.c
+++ b/src/conf/capabilities.c
@@ -948,7 +948,8 @@ virCapabilitiesFormatXML(virCapsPtr caps)
                     STREQ(caps->guests[i]->features[j]->name, "ia64_be") ||
                     STREQ(caps->guests[i]->features[j]->name, "cpuselection") ||
                     STREQ(caps->guests[i]->features[j]->name, "deviceboot") ||
-                    STREQ(caps->guests[i]->features[j]->name, "drivediscard")) {
+                    STREQ(caps->guests[i]->features[j]->name, "drivediscard") ||
+                    STREQ(caps->guests[i]->features[j]->name, "qcow2discard")) {
                     virBufferAsprintf(&xml, "      <%s/>\n",
                                       caps->guests[i]->features[j]->name);
                 } else {
diff --git a/src/qemu/qemu_capabilities.c b/src/qemu/qemu_capabilities.c
index b755fb2..b527690 100644
--- a/src/qemu/qemu_capabilities.c
+++ b/src/qemu/qemu_capabilities.c
@@ -251,6 +251,7 @@ VIR_ENUM_IMPL(virQEMUCaps, QEMU_CAPS_LAST,
               "spiceport",
 
               "usb-kbd", /* 165 */
+              "qcow2-discard",
     );
 
 struct _virQEMUCaps {
@@ -782,6 +783,10 @@ virQEMUCapsInitGuest(virCapsPtr caps,
         !virCapabilitiesAddGuestFeature(guest, "drivediscard", 0, 0))
         goto error;
 
+    if (virQEMUCapsGet(qemubinCaps, QEMU_CAPS_QCOW2_DISCARD) &&
+        !virCapabilitiesAddGuestFeature(guest, "qcow2discard", 0, 0))
+        goto error;
+
     if (virCapabilitiesAddGuestDomain(guest,
                                       "qemu",
                                       NULL,
@@ -1113,6 +1118,10 @@ virQEMUCapsComputeCmdFlags(const char *help,
     if (strstr(help, "-machine"))
         virQEMUCapsSet(qemuCaps, QEMU_CAPS_MACHINE_OPT);
 
+    /* discard option works for qcow2 backing files since 1.6 */
+    if (qemuCaps->version >= 1006000)
+        virQEMUCapsSet(qemuCaps, QEMU_CAPS_QCOW2_DISCARD);
+
      /* USB option is supported v1.3.0 onwards */
     if (qemuCaps->version >= 1003000)
         virQEMUCapsSet(qemuCaps, QEMU_CAPS_MACHINE_USB_OPT);
@@ -2569,6 +2578,10 @@ virQEMUCapsInitQMPMonitor(virQEMUCapsPtr qemuCaps,
     if (virQEMUCapsInitArchQMPBasic(qemuCaps, mon) < 0)
         goto cleanup;
 
+    /* discard option works for qcow2 backing files since 1.6 */
+    if (qemuCaps->version >= 1006000)
+        virQEMUCapsSet(qemuCaps, QEMU_CAPS_QCOW2_DISCARD);
+
     /* USB option is supported v1.3.0 onwards */
     if (qemuCaps->version >= 1003000)
         virQEMUCapsSet(qemuCaps, QEMU_CAPS_MACHINE_USB_OPT);
diff --git a/src/qemu/qemu_capabilities.h b/src/qemu/qemu_capabilities.h
index b5445e7..a0ce506 100644
--- a/src/qemu/qemu_capabilities.h
+++ b/src/qemu/qemu_capabilities.h
@@ -204,6 +204,7 @@ enum virQEMUCapsFlags {
     QEMU_CAPS_SPICE_FILE_XFER_DISABLE = 163, /* -spice disable-agent-file-xfer */
     QEMU_CAPS_CHARDEV_SPICEPORT  = 164, /* -chardev spiceport */
     QEMU_CAPS_DEVICE_USB_KBD     = 165, /* -device usb-kbd */
+    QEMU_CAPS_QCOW2_DISCARD      = 166, /* discard works for qcow2 backing files */
 
     QEMU_CAPS_LAST,                   /* this must always be the last item */
 };
diff --git a/tests/qemucapabilitiesdata/caps_1.6.0-1.caps b/tests/qemucapabilitiesdata/caps_1.6.0-1.caps
index 597f873..0ab5e22 100644
--- a/tests/qemucapabilitiesdata/caps_1.6.0-1.caps
+++ b/tests/qemucapabilitiesdata/caps_1.6.0-1.caps
@@ -142,4 +142,5 @@
     <flag name='spice-file-xfer-disable'/>
     <flag name='spiceport'/>
     <flag name='usb-kbd'/>
+    <flag name='qcow2-discard'/>
   </qemuCaps>
diff --git a/tests/qemucapabilitiesdata/caps_1.6.50-1.caps b/tests/qemucapabilitiesdata/caps_1.6.50-1.caps
index 0c1dd87..d5cca41 100644
--- a/tests/qemucapabilitiesdata/caps_1.6.50-1.caps
+++ b/tests/qemucapabilitiesdata/caps_1.6.50-1.caps
@@ -140,4 +140,5 @@
     <flag name='spice-file-xfer-disable'/>
     <flag name='spiceport'/>
     <flag name='usb-kbd'/>
+    <flag name='qcow2-discard'/>
   </qemuCaps>
-- 
1.8.5.3