[Qemu-devel] [RFC Patch 3/7]Qemu: Cmd "block_set_hostcache" for dynamic cache change

[Supriya Kannery <supriyak@linux.vnet.ibm.com>]

New command "block_set_hostcache" added for dynamically changing 
host pagecache setting of a block device.

Usage: 
 block_set_hostcache  <device> <option>
   <device> = block device
   <option> = on/off

Example:
 (qemu) block_set_hostcache ide0-hd0 off

Signed-off-by: Supriya Kannery <supriyak@linux.vnet.ibm.com>

---
 block.c         |   54 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 block.h         |    2 ++
 blockdev.c      |   26 ++++++++++++++++++++++++++
 blockdev.h      |    2 ++
 hmp-commands.hx |   14 ++++++++++++++
 qmp-commands.hx |   27 +++++++++++++++++++++++++++
 6 files changed, 125 insertions(+)

Index: qemu/block.c
===================================================================
--- qemu.orig/block.c
+++ qemu/block.c
@@ -808,6 +808,35 @@ unlink_and_fail:
     return ret;
 }
 
+int bdrv_reopen(BlockDriverState *bs, int bdrv_flags)
+{
+    BlockDriver *drv = bs->drv;
+    int ret = 0, open_flags;
+
+    /* Quiesce IO for the given block device */
+    qemu_aio_flush();
+    ret = bdrv_flush(bs);
+    if (ret != 0) {
+        qerror_report(QERR_DATA_SYNC_FAILED, bs->device_name);
+        return ret;
+    }
+    open_flags = bs->open_flags;
+    bdrv_close(bs);
+
+    ret = bdrv_open(bs, bs->filename, bdrv_flags, drv);
+    if (ret < 0) {
+        /* Reopen failed. Try to open with original flags */
+        qerror_report(QERR_REOPEN_FILE_FAILED, bs->filename);
+        ret = bdrv_open(bs, bs->filename, open_flags, drv);
+        if (ret < 0) {
+            /* Reopen failed with orig and modified flags */
+            abort();
+        }
+    }
+
+    return ret;
+}
+
 void bdrv_close(BlockDriverState *bs)
 {
     if (bs->drv) {
@@ -870,6 +899,33 @@ void bdrv_drain_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;
+    }
+
+    /* If no change in flags, no need to reopen */
+    if (bdrv_flags == bs->open_flags) {
+        return 0;
+    }
+
+    if (bdrv_is_inserted(bs)) {
+        /* Reopen file with changed set of flags */
+        bdrv_flags &= ~BDRV_O_CACHE_WB;
+        return bdrv_reopen(bs, bdrv_flags);
+    } else {
+        /* Save hostcache change for future use */
+        bs->open_flags = bdrv_flags;
+        return 0;
+    }
+}
+
 /* 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
@@ -119,6 +119,7 @@ int bdrv_parse_cache_flags(const char *m
 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_dev(BlockDriverState *bs, void *dev);
 void bdrv_attach_dev_nofail(BlockDriverState *bs, void *dev);
@@ -160,6 +161,7 @@ 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.c
===================================================================
--- qemu.orig/blockdev.c
+++ qemu/blockdev.c
@@ -1080,3 +1080,29 @@ BlockJobInfoList *qmp_query_block_jobs(E
     bdrv_iterate(do_qmp_query_block_jobs_one, &prev);
     return dummy.next;
 }
+
+
+/*
+ * Change host page cache setting while guest is running.
+*/
+int do_block_set_hostcache(Monitor *mon, const QDict *qdict,
+                           QObject **ret_data)
+{
+    BlockDriverState *bs = NULL;
+    int enable;
+    const char *device;
+
+    /* Validate device */
+    device = qdict_get_str(qdict, "device");
+    bs = bdrv_find(device);
+    if (!bs) {
+        qerror_report(QERR_DEVICE_NOT_FOUND, device);
+        return -1;
+    }
+
+    /* Read hostcache setting */
+    enable = qdict_get_bool(qdict, "option");
+    return bdrv_change_hostcache(bs, enable);
+
+}
+
Index: qemu/blockdev.h
===================================================================
--- qemu.orig/blockdev.h
+++ qemu/blockdev.h
@@ -62,4 +62,6 @@ void qmp_change_blockdev(const char *dev
                          bool has_format, const char *format, Error **errp);
 void do_commit(Monitor *mon, const QDict *qdict);
 int do_drive_del(Monitor *mon, const QDict *qdict, QObject **ret_data);
+int do_block_set_hostcache(Monitor *mon, const QDict *qdict,
+                           QObject **ret_data);
 #endif
Index: qemu/hmp-commands.hx
===================================================================
--- qemu.orig/hmp-commands.hx
+++ qemu/hmp-commands.hx
@@ -796,6 +796,20 @@ ETEXI
 	.mhandler.cmd_new = do_migrate,
     },
 
+    {
+        .name       = "block_set_hostcache",
+        .args_type  = "device:B,option:b",
+        .params     = "device on|off",
+        .help       = "Change setting of host pagecache",
+        .user_print = monitor_user_noop,
+        .mhandler.cmd_new = do_block_set_hostcache,
+    },
+STEXI
+@item block_set_hostcache @var{device} @var{option}
+@findex block_set_hostcache
+Change host pagecache setting of a block device while guest is running.
+ETEXI
+
 
 STEXI
 @item migrate [-d] [-b] [-i] @var{uri}
Index: qemu/qmp-commands.hx
===================================================================
--- qemu.orig/qmp-commands.hx
+++ qemu/qmp-commands.hx
@@ -698,7 +698,34 @@ Example:
 
 EQMP
 
+
     {
+        .name       = "block_set_hostcache",
+        .args_type  = "device:B,option:b",
+        .params     = "device on|off",
+        .help       = "Change setting of host pagecache (true|false)",
+        .user_print = monitor_user_noop,
+        .mhandler.cmd_new = do_block_set_hostcache,
+    },
+
+SQMP
+block_set_hostcache
+-------------------
+
+Change host pagecache setting of a block device (on|off)
+
+Arguments:
+
+- "device": the device's ID, must be unique (json-string)
+- "option": hostcache setting (json-bool)
+
+Example:
+-> { "execute": "block_set_hostcache", "arguments": { "device": "ide0-hd0", "option": false } }
+<- { "return": {} }
+
+EQMP
+
+	{
         .name       = "balloon",
         .args_type  = "value:M",
         .mhandler.cmd_new = qmp_marshal_input_balloon,