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



Stefano,

--On 29 March 2013 17:19:26 +0000 Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx> wrote:

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.

Alex, do you have any updates on this patch?

Sorry, I've been snowed under with other stuff.

I got to the speed bump below, then didn't get back to it after Paolo's
reply. I'll see if I can get some time tomorrow.

--
Alex Bligh

---------- Forwarded Message ----------
Date: 20 March 2013 10:26:21 +0100
From: Paolo Bonzini <pbonzini@xxxxxxxxxx>
To: Alex Bligh <alex@xxxxxxxxxxx>
CC: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Ian Campbell <Ian.Campbell@xxxxxxxxxx>, Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>, qemu-devel@xxxxxxxxxx, xen-devel <xen-devel@xxxxxxxxxxxxx>, Anthony Liguori <anthony@xxxxxxxxxxxxx> Subject: Re: [Xen-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes.

Il 20/03/2013 09:33, Alex Bligh ha scritto:
Stefano,

--On 19 March 2013 15:13:29 +0000 Stefano Stabellini
<stefano.stabellini@xxxxxxxxxxxxx> 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



---------- End Forwarded Message ----------


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