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Re: [Xen-devel] [RFC Patch v4 3/9] pass correct file to qemu if we use blktap2

On Sat, 2014-12-13 at 17:06 +0000, Wei Liu wrote:
> On Thu, Dec 11, 2014 at 04:45:09PM +0000, George Dunlap wrote:
> > On Mon, Sep 22, 2014 at 6:59 AM, Wen Congyang <wency@xxxxxxxxxxxxxx> wrote:
> > > If we use blktap2, the correct file should be blktap device
> > > not the pdev_path.
> > >
> > > Signed-off-by: Wen Congyang <wency@xxxxxxxxxxxxxx>
> > > Cc: Shriram Rajagopalan <rshriram@xxxxxxxxx>
> > > Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> > 
> > If I'm reading this correctly, this is actually a fairly serious bug
> > in libxl -- it means that when using backend=tap with HVM domains,
> > qemu will actually *bypass entirely* the tapdisk process and access
> > the underlying file directly.
> > 
> Is it because qemu will corrupt the underlying file so it's very
> serious? 

I think it ends up being reasonably safe due to the unplug stuff, in
general only one of the two paths should be active at any given time and
there is appropriate flushing/syncing on the unplug etc.

In fact, I'm not 100% sure this wasn't a deliberate design decision at
one point to avoid the overhead of doing both qdisk and tapdisk. (I'm
not going to argue that this still makes sense though).

If qemu is corrupting files then that's a different matter, which would
indeed be serious, and which this change might paper over/avoid.


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