Re: [Xen-devel] [PATCH] xencommons: Attempt to load blktap driver

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2012-05-15 at 17:21 +0100, George Dunlap wrote:
> On 15/05/12 17:16, Jan Beulich wrote:
> >>>> On 15.05.12 at 17:49, George Dunlap<george.dunlap@xxxxxxxxxxxxx>  wrote:
> >> Older kernels, such as those found in Debian Squeeze:
> >> * Have bugs in handling of AIO into foreign pages
> >> * Have blktap modules, which will cause qemu not to use AIO, but
> >> which are not loaded on boot.
> >>
> >> Attempt to load blktap in xencommons, to make sure modern qemu's which
> >> use AIO will work properly on those kernels.
> >>
> >> Signed-off-by: George Dunlap<george.dunlap@xxxxxxxxxxxxx>
> >>
> >> diff -r 99244350516a -r db614e92faf7 tools/hotplug/Linux/init.d/xencommons
> >> --- a/tools/hotplug/Linux/init.d/xencommons        Tue May 15 16:48:49 
> >> 2012 +0100
> >> +++ b/tools/hotplug/Linux/init.d/xencommons        Tue May 15 16:49:32 
> >> 2012 +0100
> >> @@ -59,6 +59,7 @@ do_start () {
> >>    modprobe evtchn 2>/dev/null
> >>    modprobe gntdev 2>/dev/null
> >>    modprobe xen-acpi-processor 2>/dev/null
> >> +  modprobe blktap 2>/dev/null
> > Can we stop manually loading all kinds of drivers here? I was
> > glad this went away with the switch to xencommons, and
> > now this is coming back. Drivers definitely needed in all cases
> > are acceptable imo, but backend drivers should be loaded as
> > backends get created by the tools (similarly frontend drivers
> > for the local attach case, though they should get auto-loaded
> > normally anyway).
> I tend to agree with you; I did it this way because that's what was 
> suggested to me.  But I don't at the moment know enough about the 
> backend creation stuff in xl / qemu to DTRT here.

The issue preventing autoloading here is that blktap is effectively
optional and libxl tries to do a best effort search for a usable disk
backend. If blktap is loaded the libxl will choose it, otherwise it will
fallback to qdisk. The problem is that if blktap is available but not
loaded then that is something which libxl cannot detect, I'm not sure
how we could go about fixing that.

Ian.



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