[Qemu-devel] [PATCH 3/5] Write table offset and size in one syscall.

[Gleb Natapov <gleb@redhat.com>]

Otherwise if VM is killed between two writes data may be lost.
But if offset and size fields are at the same disk block one
write should update them both simultaneously.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
---

 block-qcow2.c |   27 ++++++++++-----------------
 1 files changed, 10 insertions(+), 17 deletions(-)

diff --git a/block-qcow2.c b/block-qcow2.c
index 69f6414..7f99921 100644
--- a/block-qcow2.c
+++ b/block-qcow2.c
@@ -429,8 +429,7 @@ static int grow_l1_table(BlockDriverState *bs, int min_size)
     int new_l1_size, new_l1_size2, ret, i;
     uint64_t *new_l1_table;
     uint64_t new_l1_table_offset;
-    uint64_t data64;
-    uint32_t data32;
+    uint8_t data[12];
 
     new_l1_size = s->l1_size;
     if (min_size <= new_l1_size)
@@ -460,14 +459,12 @@ static int grow_l1_table(BlockDriverState *bs, int min_size)
         new_l1_table[i] = be64_to_cpu(new_l1_table[i]);
 
     /* set new table */
-    data64 = cpu_to_be64(new_l1_table_offset);
-    if (bdrv_pwrite(s->hd, offsetof(QCowHeader, l1_table_offset),
-                    &data64, sizeof(data64)) != sizeof(data64))
-        goto fail;
-    data32 = cpu_to_be32(new_l1_size);
-    if (bdrv_pwrite(s->hd, offsetof(QCowHeader, l1_size),
-                    &data32, sizeof(data32)) != sizeof(data32))
+    *(uint32_t*)data = cpu_to_be32(new_l1_size);
+    *(uint64_t*)&data[4] = cpu_to_be64(new_l1_table_offset);
+    if (bdrv_pwrite(s->hd, offsetof(QCowHeader, l1_size), data,
+                sizeof(data)) != sizeof(data))
         goto fail;
+
     qemu_free(s->l1_table);
     free_clusters(bs, s->l1_table_offset, s->l1_size * sizeof(uint64_t));
     s->l1_table_offset = new_l1_table_offset;
@@ -2278,8 +2275,7 @@ static int grow_refcount_table(BlockDriverState *bs, int min_size)
     int new_table_size, new_table_size2, refcount_table_clusters, i, ret;
     uint64_t *new_table;
     int64_t table_offset;
-    uint64_t data64;
-    uint32_t data32;
+    uint8_t data[12];
     int old_table_size;
     int64_t old_table_offset;
 
@@ -2318,13 +2314,10 @@ static int grow_refcount_table(BlockDriverState *bs, int min_size)
     for(i = 0; i < s->refcount_table_size; i++)
         be64_to_cpus(&new_table[i]);
 
-    data64 = cpu_to_be64(table_offset);
+    *(uint64_t*)data = cpu_to_be64(table_offset);
+    *(uint32_t*)&data[8] = cpu_to_be32(refcount_table_clusters);
     if (bdrv_pwrite(s->hd, offsetof(QCowHeader, refcount_table_offset),
-                    &data64, sizeof(data64)) != sizeof(data64))
-        goto fail;
-    data32 = cpu_to_be32(refcount_table_clusters);
-    if (bdrv_pwrite(s->hd, offsetof(QCowHeader, refcount_table_clusters),
-                    &data32, sizeof(data32)) != sizeof(data32))
+                    data, sizeof(data)) != sizeof(data))
         goto fail;
     qemu_free(s->refcount_table);
     old_table_offset = s->refcount_table_offset;