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

From: Kevin Wolf <kwolf@redhat.com>
To: Paul Durrant <Paul.Durrant@citrix.com>
Cc: "qemu-block@nongnu.org" <qemu-block@nongnu.org>,
	"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>,
	"xen-devel@lists.xen.org" <xen-devel@lists.xen.org>,
	Fabio Fantoni <fabio.fantoni@m2r.biz>,
	Stefano Stabellini <Stefano.Stabellini@citrix.com>,
	Anthony Perard <anthony.perard@citrix.com>,
	John Snow <jsnow@redhat.com>
Subject: Re: [Qemu-devel] Question about xen disk unplug support for ahci missed in qemu
Date: Fri, 16 Oct 2015 16:04:16 +0200	[thread overview]
Message-ID: <20151016140416.GD4185@noname.redhat.com> (raw)
In-Reply-To: <9AAE0902D5BC7E449B7C8E4E778ABCD02F6114DD@AMSPEX01CL01.citrite.net>

Am 14.10.2015 um 14:48 hat Paul Durrant geschrieben:
> > -----Original Message-----
> > From: Fabio Fantoni [mailto:fabio.fantoni@m2r.biz]
> > Sent: 14 October 2015 12:12
> > To: Kevin Wolf; Stefano Stabellini
> > Cc: John Snow; Anthony Perard; qemu-devel@nongnu.org; xen-
> > devel@lists.xen.org; qemu-block@nongnu.org; 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.

Kevin