Re: the >1Tb block issue

[Avi Kivity <avi@redhat.com>]

On 05/19/2010 11:57 AM, Christoph Hellwig wrote:
> On Tue, May 18, 2010 at 08:38:22PM +0300, Avi Kivity wrote:
>> Yes.  Why would Linux post overlapping requests? makes
>> 0xffffffff00000000 sense.
>>
>> There may be a guest bug in here too.  Christoph?
>
> Overlapping writes are entirely fine from the guest POV, although they
> should be rather unusual.  We can update a page and send it out again
> when it gets redirtied while still out on the wire.

But the device may reorder requests:

   system                              device

   issue request r1 for sector n page p
                                       dma into buffer b1
   modify contents of page p
   issue request r2 for sector n page p
                                       dma into buffer b2
                                       complete r2
                                       complete r1

Is there any guarantee r2 will complete after r1, or that b1 and b2 are 
coherent?  I'm not aware of any.

What about NFS O_DIRECT backing virtio-blk?  Here, requests can 
definitely be reordered, and the buffers are certainly not coherent 
(since they're don't even exist once the data has left the NIC).

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.