[Qemu-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes.

[Alex Bligh <alex@alex.org.uk>]

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.

This brings PV devices in line with emulated devices. Removing
O_DIRECT is safe as barrier operations are now correctly passed
through.

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

This patch has already been applied to the xenbits.org
qemu-upstream repository.
  http://xenbits.xen.org/gitweb/?p=qemu-upstream-unstable.git;a=commit;h=f3903bbac78a81fcbce1350cdce860764a62783a
Clearly it should go into qemu's own repository.

Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Alex Bligh <alex@alex.org.uk>
---
 hw/xen_disk.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/hw/xen_disk.c b/hw/xen_disk.c
index a402ac8..14f8723 100644
--- a/hw/xen_disk.c
+++ b/hw/xen_disk.c
@@ -603,7 +603,7 @@ static int blk_init(struct XenDevice *xendev)
     }
 
     /* read-only ? */
-    qflags = BDRV_O_NOCACHE | BDRV_O_CACHE_WB | BDRV_O_NATIVE_AIO;
+    qflags = BDRV_O_CACHE_WB | BDRV_O_NATIVE_AIO;
     if (strcmp(blkdev->mode, "w") == 0) {
         qflags |= BDRV_O_RDWR;
     } else {
-- 
1.7.4.1