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Date: Wed, 20 Mar 2013 10:26:21 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [Xen-devel] [PATCHv3] QEMU(upstream): Disable
 xen's use of O_DIRECT by default as it results in crashes.
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To: Alex Bligh <alex@alex.org.uk>
Cc: Ian Campbell <Ian.Campbell@citrix.com>, Stefano Stabellini <stefano.stabellini@eu.citrix.com>, George Dunlap <George.Dunlap@eu.citrix.com>, Ian Jackson <Ian.Jackson@eu.citrix.com>, qemu-devel@nongnu.org, xen-devel <xen-devel@lists.xen.org>, Anthony Liguori <anthony@codemonkey.ws>

Il 20/03/2013 09:33, Alex Bligh ha scritto:
> Stefano,
> 
> --On 19 March 2013 15:13:29 +0000 Stefano Stabellini
> <stefano.stabellini@eu.citrix.com> 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