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

[Stefano Stabellini]

On Mon, 19 Oct 2015, John Snow wrote:
> On 10/19/2015 07:44 AM, Stefano Stabellini wrote:
> > On Mon, 19 Oct 2015, Gerd Hoffmann wrote:
> >>   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?
> > 
> > I agree with you that the current unplug in nasty. Also I don't care
> > much about AHCI, in fact I don't think we should be spending efforts
> > into making that scenario work better. I think we should be working on
> > OVMF instead and fix the bug about empty cdrom drives reported by Fabio.
> > 
> 
> OVMF and AHCI go hand in hand here from my viewpoint. I'm happy to debug
> any OVMF+SATA/AHCI problems that are reported.
> 
> Last I saw, Laszlo asked Fabio for some more information on this
> problem, so I am waiting for that information to start work on that issue.

Fabio reported a bug using OVFM+xen_disk, no AHCI involved. OVFM has
already support for the Xen PV disk protocol, see
OvmfPkg/XenPvBlkDxe/XenPvBlkDxe.c.