From 38f1b9727631760b89a3c4d333e04b5d3791f325 Mon Sep 17 00:00:00 2001
From: Eduardo Habkost <ehabkost@redhat.com>
Date: Wed, 9 Sep 2009 15:57:40 -0300
Subject: [PATCH 4/5] alloc_cluster_link_l2: Write complete sectors

Message-id: <1250675987-18401-5-git-send-email-kwolf@redhat.com>
RH-Author: Kevin Wolf <kwolf@redhat.com>
Patchwork-id: 3273
O-Subject: [RHEL 5.5 PATCH 4/5] alloc_cluster_link_l2: Write complete sectors
Bugzilla: 518169
RH-Acked-by: Juan Quintela <quintela@redhat.com>
RH-Acked-by: Andrea Arcangeli <aarcange@redhat.com>
RH-Acked-by: Gleb Natapov <gleb@redhat.com>

When updating the L2 tables in alloc_cluster_link_l2(), write complete
sectors instead of updating single entries.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
---
 qemu/block-qcow2.c |   28 +++++++++++++++++++++++++---
 1 files changed, 25 insertions(+), 3 deletions(-)

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
---
 qemu/block-qcow2.c |   28 +++++++++++++++++++++++++---
 1 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/qemu/block-qcow2.c b/qemu/block-qcow2.c
index 9c8212b..50561f1 100644
--- a/qemu/block-qcow2.c
+++ b/qemu/block-qcow2.c
@@ -1005,6 +1005,28 @@ typedef struct QCowL2Meta
     int nb_clusters;
 } QCowL2Meta;
 
+/*
+ * Write L2 table updates to disk, writing whole sectors to avoid a
+ * read-modify-write in bdrv_pwrite
+ */
+#define L2_ENTRIES_PER_SECTOR (512 / 8)
+static int write_l2_entries(BDRVQcowState *s, uint64_t *l2_table,
+    uint64_t l2_offset, int l2_index, int num)
+{
+    int l2_start_index = l2_index & ~(L1_ENTRIES_PER_SECTOR - 1);
+    int start_offset = (8 * l2_index) & ~511;
+    int end_offset = (8 * (l2_index + num) + 511) & ~511;
+    size_t len = end_offset - start_offset;
+
+    if (bdrv_pwrite(s->hd, l2_offset + start_offset, &l2_table[l2_start_index],
+        len) != len)
+    {
+        return -1;
+    }
+
+    return 0;
+}
+
 static int alloc_cluster_link_l2(BlockDriverState *bs, uint64_t cluster_offset,
         QCowL2Meta *m)
 {
@@ -1053,10 +1075,10 @@ static int alloc_cluster_link_l2(BlockDriverState *bs, uint64_t cluster_offset,
                     (i << s->cluster_bits)) | QCOW_OFLAG_COPIED);
      }
 
-    if (bdrv_pwrite(s->hd, l2_offset + l2_index * sizeof(uint64_t),
-                l2_table + l2_index, m->nb_clusters * sizeof(uint64_t)) !=
-            m->nb_clusters * sizeof(uint64_t))
+    if (write_l2_entries(s, l2_table, l2_offset, l2_index, m->nb_clusters) < 0) {
+        ret = -1;
         goto err;
+    }
 
     for (i = 0; i < j; i++)
         free_any_clusters(bs, be64_to_cpu(old_cluster[i]) & ~QCOW_OFLAG_COPIED,
-- 
1.6.3.rc4.29.g8146