The patch add support L2 table cache for qcow2 disk. L2 table cache can improve IO read and write performance for qcow2 img.
Diff follows:
 src/conf/domain_conf.c                               | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 src/conf/domain_conf.h                               |  5 +++++
 src/qemu/qemu_command.c                              |  7 +++++++
 src/qemu/qemu_domain.c                               |  6 ++++++
 tests/qemuxml2argvdata/qemuxml2argv-disk-virtio.args |  2 +-
 tests/qemuxml2argvdata/qemuxml2argv-disk-virtio.xml  |  3 ++-
 6 files changed, 68 insertions(+), 2 deletions(-)


diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
index beca0be..0a6afca 100644
--- a/src/conf/domain_conf.c
+++ b/src/conf/domain_conf.c
@@ -8058,6 +8058,8 @@ virDomainDiskDefParseXML(virDomainXMLOptionPtr xmlopt,
     char *domain_name = NULL;
     int expected_secret_usage = -1;
     int auth_secret_usage = -1;
+    char *disk_l2_cache_size = NULL;
+    char *disk_cache_clean_interval = NULL;

     if (!(def = virDomainDiskDefNew(xmlopt)))
         return NULL;
@@ -8233,6 +8235,27 @@ virDomainDiskDefParseXML(virDomainXMLOptionPtr xmlopt,
             }
         } else if (xmlStrEqual(cur->name, BAD_CAST "boot")) {
             /* boot is parsed as part of virDomainDeviceInfoParseXML */
+        } else if (xmlStrEqual(cur->name, BAD_CAST "diskCache")) {
+            disk_l2_cache_size =
+                virXMLPropString(cur, "disk_l2_cache_size");
+            if (disk_l2_cache_size &&
+                virStrToLong_ui(disk_l2_cache_size, NULL, 0,
+                                &def->disk_cache.disk_l2_cache_size) < 0) {
+                virReportError(VIR_ERR_INTERNAL_ERROR,
+                               _("invalid disk L2 cache size '%s'"),
+                               disk_l2_cache_size);
+                goto error;
+            }
+            disk_cache_clean_interval =
+                virXMLPropString(cur, "disk_cache_clean_interval");
+            if (disk_cache_clean_interval &&
+                virStrToLong_ui(disk_cache_clean_interval, NULL, 0,
+                                &def->disk_cache.disk_cache_clean_interval) < 0) {
+                virReportError(VIR_ERR_INTERNAL_ERROR,
+                               _("invalid disk cache clean interval '%s'"),
+                               disk_cache_clean_interval);
+                goto error;
+            }
         }
     }

@@ -8472,6 +8495,8 @@ virDomainDiskDefParseXML(virDomainXMLOptionPtr xmlopt,
     VIR_FREE(vendor);
     VIR_FREE(product);
     VIR_FREE(domain_name);
+    VIR_FREE(disk_l2_cache_size);
+    VIR_FREE(disk_cache_clean_interval);

     ctxt->node = save_ctxt;
     return def;
@@ -20646,6 +20671,27 @@ virDomainDiskBlockIoDefFormat(virBufferPtr buf,
     }
 }

+static void
+virDomainDiskCacheDefFormat(virBufferPtr buf,
+                              virDomainDiskDefPtr def)
+{
+    if (def->disk_cache.disk_l2_cache_size > 0 ||
+        def->disk_cache.disk_cache_clean_interval > 0) {
+        virBufferAddLit(buf, "<diskCache");
+        if (def->disk_cache.disk_l2_cache_size > 0) {
+            virBufferAsprintf(buf,
+                              " disk_l2_cache_size='%u'",
+                              def->disk_cache.disk_l2_cache_size);
+        }
+        if (def->disk_cache.disk_cache_clean_interval > 0) {
+            virBufferAsprintf(buf,
+                              " disk_cache_clean_interval='%u'",
+                              def->disk_cache.disk_cache_clean_interval);
+        }
+        virBufferAddLit(buf, "/>\n");
+    }
+}
+

 /* virDomainSourceDefFormatSeclabel:
  *
@@ -20990,6 +21036,7 @@ virDomainDiskDefFormat(virBufferPtr buf,

     virDomainDiskGeometryDefFormat(buf, def);
     virDomainDiskBlockIoDefFormat(buf, def);
+    virDomainDiskCacheDefFormat(buf, def);

     /* For now, mirroring is currently output-only: we only output it
      * for live domains, therefore we ignore it on input except for
diff --git a/src/conf/domain_conf.h b/src/conf/domain_conf.h
index 91a33cb..83ea9d3 100644
--- a/src/conf/domain_conf.h
+++ b/src/conf/domain_conf.h
@@ -647,6 +647,11 @@ struct _virDomainDiskDef {
         unsigned int physical_block_size;
     } blockio;

+    struct {
+        unsigned int disk_l2_cache_size;
+        unsigned int disk_cache_clean_interval;
+    } disk_cache;
+
     virDomainBlockIoTuneInfo blkdeviotune;

     char *serial;
diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
index e087891..7c47cd3 100755
--- a/src/qemu/qemu_command.c
+++ b/src/qemu/qemu_command.c
@@ -1662,6 +1662,13 @@ qemuBuildDriveStr(virDomainDiskDefPtr disk,
     if (qemuBuildDriveSourceStr(disk, cfg, &opt, qemuCaps) < 0)
         goto error;

+    if (disk->disk_cache.disk_l2_cache_size > 0)
+        virBufferAsprintf(&opt, "l2-cache-size=%u,",
+                disk->disk_cache.disk_l2_cache_size);
+    if (disk->disk_cache.disk_cache_clean_interval > 0)
+        virBufferAsprintf(&opt, "cache-clean-interval=%u,",
+                disk->disk_cache.disk_cache_clean_interval);
+
     if (emitDeviceSyntax)
         virBufferAddLit(&opt, "if=none");
     else
diff --git a/src/qemu/qemu_domain.c b/src/qemu/qemu_domain.c
index 9f165c1..de334a7 100644
--- a/src/qemu/qemu_domain.c
+++ b/src/qemu/qemu_domain.c
@@ -5408,6 +5408,12 @@ qemuDomainDiskChangeSupported(virDomainDiskDefPtr disk,
     CHECK_EQ(ioeventfd, "ioeventfd", true);
     CHECK_EQ(event_idx, "event_idx", true);
     CHECK_EQ(copy_on_read, "copy_on_read", true);
+
+    CHECK_EQ(disk_cache.disk_l2_cache_size,
+             "diskCache disk_l2_cache_size", true);
+    CHECK_EQ(disk_cache.disk_cache_clean_interval,
+             "diskCache disk_cache_clean_interval", true);
+
     /* "snapshot" is a libvirt internal field and thus can be changed */
     /* startupPolicy is allowed to be updated. Therefore not checked here. */
     CHECK_EQ(transient, "transient", true);
diff --git a/tests/qemuxml2argvdata/qemuxml2argv-disk-virtio.args b/tests/qemuxml2argvdata/qemuxml2argv-disk-virtio.args
index b405242..b968302 100644
--- a/tests/qemuxml2argvdata/qemuxml2argv-disk-virtio.args
+++ b/tests/qemuxml2argvdata/qemuxml2argv-disk-virtio.args
@@ -22,7 +22,7 @@ QEMU_AUDIO_DRV=none \
 -drive file=/dev/HostVG/QEMUGuest2,format=raw,if=none,media=cdrom,\
 id=drive-ide0-1-0,readonly=on \
 -device ide-drive,bus=ide.1,unit=0,drive=drive-ide0-1-0,id=ide0-1-0 \
--drive file=/tmp/data.img,format=raw,if=none,id=drive-virtio-disk0 \
+-drive file=/tmp/data.img,format=qcow2,l2-cache-size=536870912,cache-clean-interval=900,if=none,id=drive-virtio-disk0 \
 -device virtio-blk-pci,bus=pci.0,addr=0x3,drive=drive-virtio-disk0,\
 id=virtio-disk0 \
 -drive file=/tmp/logs.img,format=raw,if=none,id=drive-virtio-disk1 \
diff --git a/tests/qemuxml2argvdata/qemuxml2argv-disk-virtio.xml b/tests/qemuxml2argvdata/qemuxml2argv-disk-virtio.xml
index b843878..43298fb 100644
--- a/tests/qemuxml2argvdata/qemuxml2argv-disk-virtio.xml
+++ b/tests/qemuxml2argvdata/qemuxml2argv-disk-virtio.xml
@@ -28,8 +28,9 @@
       <address type='drive' controller='0' bus='1' target='0' unit='0'/>
     </disk>
     <disk type='file' device='disk'>
-      <driver name='qemu' type='raw'/>
+      <driver name='qemu' type='qcow2'/>
       <source file='/tmp/data.img'/>
+      <diskCache disk_l2_cache_size='53687' disk_cache_clean_interval='900' />
       <target dev='vda' bus='virtio'/>
     </disk>
     <disk type='file' device='disk'>


Thanks