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

[Alex Bligh <alex@xxxxxxxxxxx>]

Stefano, George,

> This patch only impacts the PV backend in QEMU, not the IDE interface.
> PV frontends and backends always disconnect and reconnect during
> save/restore.
> So we can be *certain* that bdrv_close at the sender side is called
> before the new connection is established at the receiver side.
>
> Unfortunately the QEMU PV backend opens the disk during initialization,
> so before the actual connection is established (blk_init).
>
> 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.
Stefano: do you want me to come up with a patch to do this?

--On 19 March 2013 15:29:36 +0000 George Dunlap 
<George.Dunlap@xxxxxxxxxxxxx> wrote:
> Alex, the title of the changelog should certainly be more specific, so
> people aren't confused.  Maybe something like the following?
>
> "Disable use of O_DIRECT when acting as a Xen PV backend"
>
> And emphasizing in the changelog that this has no effect on qemu when
> acting as a device model for Xen?
Sure, I can do that.

However, my understanding (possibly incorrect) is that when qemu is
acting as a device model for xen (i.e. emulated disks) it doesn't
use O_DIRECT anyway. I am aware this is a different problem!

--
Alex Bligh

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