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

[George Dunlap <George.Dunlap@eu.citrix.com>]

On Fri, Mar 8, 2013 at 3:18 AM, Stefano Stabellini
<stefano.stabellini@eu.citrix.com> wrote:
> On Thu, 7 Mar 2013, Alex Bligh wrote:
>> 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@alex.org.uk>
>> ---
>>  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 {
>
> I would just remove use_o_direct and BDRV_O_NOCACHE altogether

Wait, aren't O_DIRECT and BDRV_O_NOCACHE required for safety?  That
is, without these flags isn't it possible that the guest OS thinks
that the data has made it onto stable storage, while in fact it's
still in dom0's memory?  Or am I missing something?

 -George