[Xen-devel] [PATCH] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes.

[Alex Bligh <alex@xxxxxxxxxxx>]

Due to what is almost certainly a kernel bug, writes with
O_DIRECT may continue to reference the page after the write
has been marked as completed, particularly in the case of
TCP retransmit. In other scenarios, this "merely" risks
data corruption on the write, but with Xen pages from domU
are only transiently mapped into dom0's memory, resulting
in kernel panics when they are subsequently accessed.

See:
  http://lists.xen.org/archives/html/xen-devel/2012-12/msg01154.html
for more details.

Signed-off-by: Alex Bligh <alex@xxxxxxxxxxx>
---
 hw/xen_disk.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/hw/xen_disk.c b/hw/xen_disk.c
index a402ac8..a618d8d 100644
--- a/hw/xen_disk.c
+++ b/hw/xen_disk.c
@@ -45,6 +45,8 @@ static int batch_maps   = 0;
 
 static int max_requests = 32;
 
+static int use_o_direct = 0;
+
 /* ------------------------------------------------------------- */
 
 #define BLOCK_SIZE  512
@@ -603,7 +605,7 @@ static int blk_init(struct XenDevice *xendev)
     }
 
     /* read-only ? */
-    qflags = BDRV_O_NOCACHE | BDRV_O_CACHE_WB | BDRV_O_NATIVE_AIO;
+    qflags = (use_o_direct?BDRV_O_NOCACHE:0) | BDRV_O_CACHE_WB | 
BDRV_O_NATIVE_AIO;
     if (strcmp(blkdev->mode, "w") == 0) {
         qflags |= BDRV_O_RDWR;
     } else {
-- 
1.7.4.1


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