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.stabell...@eu.citrix.com> Signed-off-by: Alex Bligh <a...@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
