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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Kevin Wolf [mailto:kwolf@xxxxxxxxxx]
> Sent: 16 October 2015 15:04
> To: Paul Durrant
> Cc: Fabio Fantoni; Stefano Stabellini; John Snow; Anthony Perard; qemu-
> devel@xxxxxxxxxx; xen-devel@xxxxxxxxxxxxx; qemu-block@xxxxxxxxxx
> Subject: Re: [Qemu-devel] Question about xen disk unplug support for ahci
> missed in qemu
> 
> Am 14.10.2015 um 14:48 hat Paul Durrant geschrieben:
> > > -----Original Message-----
> > > From: Fabio Fantoni [mailto:fabio.fantoni@xxxxxxx]
> > > Sent: 14 October 2015 12:12
> > > To: Kevin Wolf; Stefano Stabellini
> > > Cc: John Snow; Anthony Perard; qemu-devel@xxxxxxxxxx; xen-
> > > devel@xxxxxxxxxxxxx; qemu-block@xxxxxxxxxx; Paul Durrant
> > > Subject: Re: [Qemu-devel] Question about xen disk unplug support for
> ahci
> > > missed in qemu
> > >
> > >
> > >
> > > Il 14/10/2015 11:47, Kevin Wolf ha scritto:
> > > > [ 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=679f4f8b178e7c66fbc2f39
> > > c905374ee8663d5d8
> > > >>>>
> > > >>>> 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.
> >
> > Unfortunately, it's going to be difficult to remove such 'craziness' when 
> > you
> don't know a priori whether the VM has PV drivers or not.
> 
> Why wouldn't you know that beforehand? I mean, even on real hardware
> you
> can have different disk interfaces (IDE, AHCI, SCSI) and you install
> the exact driver that your hardware needs. You just do the same thing on
> VM: If your hardware is PV, you install a PV driver. If your hardware is
> IDE, you install an IDE driver. Whether it's PV or IDE is something that
> you, the user, decided when configuring the VM, so you definitely know.
> 

That's not necessarily true. The host admin that provisions the VM does not 
necessarily know what OS the user of that VM will install. The admin may just 
be providing a generic VM with an emulated CD drive that the user can point at 
any ISO they want.

So, as a host admin, if you provide a VM with only PV backends and your user is 
trying to boot an OS with no PV drivers they are not going to be happy, so you 
provide emulated devices. Then, at some point later, when the user installs PV 
drivers, there really should be some way for those drivers to start up without 
any need to contact the host admin and have the VM reconfigured.

  Paul

> Kevin

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