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

[Kevin Wolf]

Am 16.10.2015 um 17:10 hat Paul Durrant geschrieben:
> > -----Original Message-----
> > From: Kevin Wolf [mailto:address@hidden
> > Sent: 16 October 2015 16:02
> > To: Paul Durrant
> > Cc: Fabio Fantoni; Stefano Stabellini; John Snow; Anthony Perard; qemu-
> > address@hidden; address@hidden; address@hidden
> > Subject: Re: [Qemu-devel] Question about xen disk unplug support for ahci
> > missed in qemu
> > 
> > Am 16.10.2015 um 16:24 hat Paul Durrant geschrieben:
> > > > -----Original Message-----
> > > > From: Kevin Wolf [mailto:address@hidden
> > > > Sent: 16 October 2015 15:04
> > > > To: Paul Durrant
> > > > Cc: Fabio Fantoni; Stefano Stabellini; John Snow; Anthony Perard; qemu-
> > > > address@hidden; address@hidden; address@hidden
> > > > 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:address@hidden
> > > > > > Sent: 14 October 2015 12:12
> > > > > > To: Kevin Wolf; Stefano Stabellini
> > > > > > Cc: John Snow; Anthony Perard; address@hidden; xen-
> > > > > > address@hidden; address@hidden; 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.
> > 
> > Why only IDE and xendisk then? Maybe I have an OS that works great with
> > AHCI, or virtio-blk, or an LSI SCSI controller, or a Megasas SCSI
> > controller, or USB sticks, or... (and IDE will hardly ever be the
> > optimal one)
> > 
> > What about network cards? My OS might support the Xen PV one, or it
> > might support rtl8139, or e1000, or virtio-net, or pcnet, or...
> > 
> > Should we always put all of the hardware that can possibly be emulated
> > in a VM just so that the one right device is definitely included even
> > though we don't know what OS will be running?
> > 
> > This is ridiculous.
> 
> It might be, but to some extent it's reality. The reason that the
> default emulated network device chosen by xl is rtl8193 is that it has
> drivers in just about every OS. The same reason for IDE being the
> default choice for storage.

So what does this mean for a justification for the AHCI + xendisk hybrid
proposal?

> > Just tell your admin what virtual hardware you really need. (Or tell
> > them to give you a proper interface to configure your VMs yourself.)
> > 
> 
> My point is that the virtual hardware that the OS user wants will
> change. Before they install PV drivers, they will need emulated
> device. After installing PV drivers they will want PV devices. Should
> they really have to contact their cloud provider to make the switch,
> when at the moment it happens automatically and transparently (the
> AHCI problem aside)?

My point is that such a magic change shouldn't happen. It doesn't happen
on real hardware either and people still get things installed to non-IDE
disks.

There is no reason to install the OS onto a different device than will
be used later. With Linux, it's no problem at all because the PV drivers
are already included on the installation media anyway, and on Windows or
presumably any other OS you can load and install the drivers right from
the beginning.

In fact, I would be surprised if using xendisk instead of IDE for
installing Windows didn't result in a noticably faster installation.


Now, if you really insist on providing a legacy interface even to guests
that eventually use PV drivers, there actually are sane ways to
implement this. It will be tricky to make that transition now without
breaking compatibility, but it could have been done from the start.

Sane means for example that you don't open the same image twice (and
even read-write!) at the same time. This is a recipe for disaster and
it's surprising that you don't see corrupted images more often.

So if you wanted to have a clean solution, try to think how real
hardware would solve the problem. If you want me to suggest something
off the top of my head, I would come up with an extended IDE device (one
single device!) that provides the IDE I/O ports and additionally some
MMIO BAR that enables access to PV functionality.

Once you enable PV functionality, the IDE ports stop working; device
reset disables the PV ring and goes back to IDE mode. No hard disk
suddenly disappearing from the machine, no image corruption if the IDE
device is written to before enabling PV, etc.


But it's your choice. You can keep your broken hack in IDE. Just don't
expect anyone to support adding new broken hacks to other devices.

Kevin