Index: qemu/hmp-commands.hx
===================================================================
--- qemu.orig/hmp-commands.hx
+++ qemu/hmp-commands.hx
@@ -70,6 +70,21 @@ but should be used with extreme caution.
 resizes image files, it can not resize block devices like LVM volumes.
 ETEXI
 
+	{
+		.name       = "block_set",
+		.args_type  = "device:B,paramname:s,enable:b",
+		.params     = "device paramname enable",
+		.help       = "On/Off block device parameters like hostcache",
+		.user_print = monitor_user_noop,
+		.mhandler.cmd_new = do_block_set,
+	},
+
+STEXI
+@item block_set
+@findex block_set
+Change block device params (eg:"hostcache"=on/off) while guest is running.
+ETEXI
+
 
     {
         .name       = "eject",
Index: qemu/qmp-commands.hx
===================================================================
--- qemu.orig/qmp-commands.hx
+++ qemu/qmp-commands.hx
@@ -693,7 +693,35 @@ Example:
 
 EQMP
 
-    {
+	{
+		.name       = "block_set",
+		.args_type  = "device:B,name:s,enable:b",
+		.params     = "device name enable",
+		.help       = "Enable/Disable block device params like hostcache",
+		.user_print = monitor_user_noop,
+		.mhandler.cmd_new = do_block_set,
+	},
+
+SQMP
+block_set
+---------
+
+Change various block device parameters like hostcache.
+
+Arguments:
+
+- "device": the device's ID, must be unique (json-string)
+- "name": name of the parameter to be changed" (json-string)
+- "enable": value to be set for the parameter, 'true' or 'false' (json-bool)
+
+Example:
+
+-> { "execute": "block_set", "arguments": { "device": "ide0-hd0", "name": "hostcache", "enable": true } }
+<- { "return": {} }
+
+EQMP
+
+	{
         .name       = "balloon",
         .args_type  = "value:M",
         .params     = "target",
Index: qemu/blockdev.c
===================================================================
--- qemu.orig/blockdev.c
+++ qemu/blockdev.c
@@ -797,3 +797,35 @@ int do_block_resize(Monitor *mon, const 
 
     return 0;
 }
+
+
+/*
+ * Handle changes to block device settings, like hostcache,
+ * while guest is running.
+*/
+int do_block_set(Monitor *mon, const QDict *qdict, QObject **ret_data)
+{
+	const char *device = qdict_get_str(qdict, "device");
+	const char *name = qdict_get_str(qdict, "name");
+	int enable = qdict_get_bool(qdict, "enable");
+	BlockDriverState *bs;
+
+	bs = bdrv_find(device);
+	if (!bs) {
+		qerror_report(QERR_DEVICE_NOT_FOUND, device);
+		return -1;
+	}
+
+	if (!(strcmp(name, "hostcache"))) {
+		if (bdrv_is_inserted(bs)) {
+			/* cache change applicable only if device inserted */
+			return bdrv_change_hostcache(bs, enable);
+		} else {
+			qerror_report(QERR_DEVICE_NOT_INSERTED, device);
+			return -1;
+		}
+	}
+
+	return 0;
+}
+
Index: qemu/block.c
===================================================================
--- qemu.orig/block.c
+++ qemu/block.c
@@ -651,6 +651,33 @@ unlink_and_fail:
     return ret;
 }
 
+int bdrv_reopen(BlockDriverState *bs, int bdrv_flags)
+{
+	BlockDriver *drv = bs->drv;
+	int ret = 0;
+
+	/* No need to reopen as no change in flags */
+	if (bdrv_flags == bs->open_flags)
+		return 0;
+
+	/* Quiesce IO for the given block device */
+	qemu_aio_flush();
+	bdrv_flush(bs);
+
+	bdrv_close(bs);
+	ret = bdrv_open(bs, bs->filename, bdrv_flags, drv);
+
+	/*
+	* A failed attempt to reopen the image file must lead to 'abort()'
+	*/
+	if (ret != 0) {
+		qerror_report(QERR_REOPEN_FILE_FAILED, bs->filename);
+		abort();
+	}
+
+	return ret;
+}
+
 void bdrv_close(BlockDriverState *bs)
 {
     if (bs->drv) {
@@ -691,6 +718,20 @@ void bdrv_close_all(void)
     }
 }
 
+int bdrv_change_hostcache(BlockDriverState *bs, bool enable_host_cache)
+{
+	int bdrv_flags = bs->open_flags;
+
+	/* set hostcache flags (without changing WCE/flush bits) */
+	if (enable_host_cache)
+		bdrv_flags &= ~BDRV_O_NOCACHE;
+	else
+		bdrv_flags |= BDRV_O_NOCACHE;
+
+	/* Reopen file with changed set of flags */
+	return bdrv_reopen(bs, bdrv_flags);
+}
+
 /* make a BlockDriverState anonymous by removing from bdrv_state list.
    Also, NULL terminate the device_name to prevent double remove */
 void bdrv_make_anon(BlockDriverState *bs)
Index: qemu/block.h
===================================================================
--- qemu.orig/block.h
+++ qemu/block.h
@@ -71,6 +71,7 @@ void bdrv_delete(BlockDriverState *bs);
 int bdrv_file_open(BlockDriverState **pbs, const char *filename, int flags);
 int bdrv_open(BlockDriverState *bs, const char *filename, int flags,
               BlockDriver *drv);
+int bdrv_reopen(BlockDriverState *bs, int bdrv_flags);
 void bdrv_close(BlockDriverState *bs);
 int bdrv_attach(BlockDriverState *bs, DeviceState *qdev);
 void bdrv_detach(BlockDriverState *bs, DeviceState *qdev);
@@ -96,6 +97,7 @@ void bdrv_commit_all(void);
 int bdrv_change_backing_file(BlockDriverState *bs,
     const char *backing_file, const char *backing_fmt);
 void bdrv_register(BlockDriver *bdrv);
+int bdrv_change_hostcache(BlockDriverState *bs, bool enable_host_cache);
 
 
 typedef struct BdrvCheckResult {
Index: qemu/blockdev.h
===================================================================
--- qemu.orig/blockdev.h
+++ qemu/blockdev.h
@@ -65,5 +65,6 @@ int do_change_block(Monitor *mon, const 
 int do_drive_del(Monitor *mon, const QDict *qdict, QObject **ret_data);
 int do_snapshot_blkdev(Monitor *mon, const QDict *qdict, QObject **ret_data);
 int do_block_resize(Monitor *mon, const QDict *qdict, QObject **ret_data);
+int do_block_set(Monitor *mon, const QDict *qdict, QObject **ret_data);
 
 #endif
