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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 14 Oct 2015, Kevin Wolf wrote:
> [ CC qemu-block ]
> 
> Am 13.10.2015 um 19:10 hat Stefano Stabellini geschrieben:
> > On Tue, 13 Oct 2015, John Snow wrote:
> > > On 10/13/2015 11:55 AM, Fabio Fantoni wrote:
> > > > I added ahci disk support in libxl and using it for week seems that was
> > > > ok, after a reply of Stefano Stabellini seems that xen disk unplug
> > > > support only ide disks:
> > > > http://git.qemu.org/?p=qemu.git;a=commitdiff;h=679f4f8b178e7c66fbc2f39c905374ee8663d5d8
> > > > 
> > > > Today Paul Durrant told me that even if pv disk is ok also with ahci and
> > > > the emulated one is offline can be a risk:
> > > > http://lists.xenproject.org/archives/html/win-pv-devel/2015-10/msg00021.html
> > > > 
> > > > 
> > > > I tried to take a fast look in qemu code but I not understand the needed
> > > > thing for add the xen disk unplug support also for ahci, can someone do
> > > > it or tell me useful information for do it please?
> > > > 
> > > > Thanks for any reply and sorry for my bad english.
> > > > 
> > > 
> > > I'm not entirely sure what features you need AHCI to support in order
> > > for Xen to be happy.
> > > 
> > > I'd guess hotplugging, but where I get confused is that IDE disks don't
> > > support hotplugging either, so I guess I'm not sure sure what you need.
> > > 
> > > Stefano, can you help bridge my Xen knowledge gap?
> >  
> > Hi John,
> > 
> > we need something like hw/i386/xen/xen_platform.c:unplug_disks but that
> > can unplug AHCI disk. And by unplug, I mean "make disappear" like
> > pci_piix3_xen_ide_unplug does for ide.
> 
> Maybe this would be the right time to stop the craziness with your
> hybrid IDE/xendisk setup. It's a horrible thing that would never happen
> on real hardware.

I would be quite happy to stop (or even get rid of) the craziness.


> Can't you just teach SeaBIOS how to deal with your PV disks and then
> only add that to your VM and forget about IDE/AHCI? I mean, that's how
> it's done for virtio-blk, and it doesn't involve any insanities like
> ripping out non-hotpluggable devices.

Teaching SeaBIOS to deal with PV disks can be done, in fact we already
support PV disks in OVMF. It is possible to boot Windows with OVMF
without any IDE disks (patch pending for libxl to create a VM without
emulated IDE disks).

However we have to be honest that implementing PV disk support in
SeaBIOS is a different magnitude of effort compared to implementing AHCI
"unplug".

I would suggest Fabio to avoid AHCI disks altogether and just use OVMF
with PV disks only and Anthony's patch to libxl to avoid creating any
IDE disks: http://marc.info/?l=xen-devel&m=144482080812353.

Would that work for you?


> Hm... How does a reboot of a machine that had its IDE already removed
> actually work? Do you restart the qemu process on reboot?

Restart QEMU, yes.

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