[Qemu-devel] [PATCH 2/4] block: add a helper to change writeback mode on the fly

[Christoph Hellwig <hch@lst.de>]

Add a new bdrv_change_cache that can set/clear the writeback flag
at runtime by stopping all I/O and closing/reopening the image file.

All code is based on a patch from Prerna Saxena <prerna@linux.vnet.ibm.com>
with minimal refactoring.

Signed-off-by: Christoph Hellwig <hch@lst.de>

Index: qemu/block.c
===================================================================
--- qemu.orig/block.c	2011-03-15 11:47:31.285634626 +0100
+++ qemu/block.c	2011-03-15 14:57:03.680633093 +0100
@@ -441,6 +441,8 @@ static int bdrv_open_common(BlockDriverS
 
     if (flags & BDRV_O_CACHE_WB)
         bs->enable_write_cache = 1;
+    else
+        bs->enable_write_cache = 0;
 
     /*
      * Clear flags that are internal to the block layer before opening the
@@ -651,6 +653,44 @@ unlink_and_fail:
     return ret;
 }
 
+static int bdrv_reopen(BlockDriverState *bs, int bdrv_flags)
+{
+    BlockDriver *drv = bs->drv;
+    int ret;
+
+    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) {
+        abort();
+    }
+
+    return ret;
+}
+
+int bdrv_change_cache(BlockDriverState *bs, bool enable)
+{
+    int bdrv_flags = 0;
+
+    bdrv_flags = bs->open_flags & ~BDRV_O_CACHE_WB;
+    if (enable) {
+        bdrv_flags |= BDRV_O_CACHE_WB;
+    }
+
+    return bdrv_reopen(bs, bdrv_flags);
+}
+
 void bdrv_close(BlockDriverState *bs)
 {
     if (bs->drv) {
Index: qemu/block.h
===================================================================
--- qemu.orig/block.h	2011-03-15 11:47:18.664136441 +0100
+++ qemu/block.h	2011-03-15 11:47:31.813634525 +0100
@@ -87,6 +87,7 @@ int bdrv_pwrite_sync(BlockDriverState *b
 int bdrv_write_sync(BlockDriverState *bs, int64_t sector_num,
     const uint8_t *buf, int nb_sectors);
 int bdrv_truncate(BlockDriverState *bs, int64_t offset);
+int bdrv_change_cache(BlockDriverState *bs, bool enable);
 int64_t bdrv_getlength(BlockDriverState *bs);
 void bdrv_get_geometry(BlockDriverState *bs, uint64_t *nb_sectors_ptr);
 void bdrv_guess_geometry(BlockDriverState *bs, int *pcyls, int *pheads, int *psecs);