Re: [Qemu-devel] Question about xen disk unplug support for ahci missed in qemu

[Gerd Hoffmann <kraxel@redhat.com>]

  Hi,

> > I'm trying to follow this discussion as best as I am able, but my lack
> > of experience with Xen prevents me from really participating in a
> > meaningful way.
> > 
> > (I see that Laszlo is still discussing some CD-ROM issues with Fabio
> > which may be of interest to me...)
> > 
> > At any rate, I won't be authoring any Xen-specific hacks to the AHCI
> > device, but I do have plans to implement hot-plugging emulation as per
> > the AHCI spec. Perhaps this is sufficient for the Xen layer, but someone
> > else will need to author the appropriate glue code.
> > 
> > If "real" hot-plugging is not sufficient, we'll need to discuss further,
> > preferably over some RFC patches.
> 
> That's fine. AHCI hot-plugging would go a long way and once we have
> that, the rest is easy.

Can we get some more background on this?

IIRC the IDE bits are needed to boot hvm guests, which goes like this:

  (1) boot disk is hooked up using both xenbus and ide.
  (2) seabios boots using ide.
  (3) linux kernel activates xenbus, at which point qemu zaps the ide
      disks to avoid the disk being present twice in the system.

Correct?

Do we really want repeat this exercise for AHCI?  Alot has changed since
this boot hack for ide was added ...

As far I know OVMF has xenbus drivers, so OVMF should already boot xen
guests just fine without this, correct?

Can we just have xenbus drivers for seabios too?  seabios can run disk
drivers in 32bit mode meanwhile, so this should not be as difficult any
more as it used to be.

cheers,
  Gerd