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

[Paolo Bonzini]

Il 20/03/2013 09:33, Alex Bligh ha scritto:
> Stefano,
> 
> --On 19 March 2013 15:13:29 +0000 Stefano Stabellini
> <address@hidden> wrote:
> 
>> Therefore I think that the current change is not safe, but it is pretty
>> easy to make it safe.
>> You just need to move the call to blk_open from blk_init to blk_connect.
>> Actually you can move most of blk_init to blk_connect, you just need to
>> keep the 4 xenstore_write_be_int at the end of the function.
> 
> The final of these 4 writes in blk_init is:
> 
>    xenstore_write_be_int(&blkdev->xendev, "sectors",
>                          blkdev->file_size / blkdev->file_blk);
> 
> and blkdev->filesize comes from bdrv_getlength(blkdev->bs), and that
> requires (I think) bdrv_open to have been called. Can these
> 4 writes move safely to blk_connect? Or can we write the sectors
> count as (say) 0 here and fix it later in blk_connect? The remainder
> look ok.

I think so.  blkfront reads "sectors" when QEMU moves to
XenbusStateConnected, in blkfront_connect.

blk_connect is called from xen_be_try_initialise, which moves to
XenbusStateConnected on success.  So, QEMU's blk_connect will always be
called before blkfront's blkfront_connect.

Paolo