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

[Stefano Stabellini]

On Wed, 14 Oct 2015, Kevin Wolf wrote:
> Am 14.10.2015 um 13:06 hat Stefano Stabellini geschrieben:
> > 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".
> 
> Agreed. But we generally try to do the right thing and not the easy
> thing.

Sure. I am quite happy to let other people do it though :-)


> Maybe I'm missing something, but my impression was that the hybrid setup
> was only used so that you can boot from a PV disk even though the BIOS
> doesn't have a PV driver. In that case, why would you even want to use
> AHCI instead of IDE? During the early boot performance shouldn't be much
> different as there is no parallelism, and afterwards the PV driver is
> used.

The "unplug" code and the design choice predate me -- I am not sure why
it was done like that.  In those days there was no SeaBIOS: maybe they
thought that writing a PV disk frontend in rombios would be madness? 

In addition isn't it true that some guests, once they see a PIIX3
chipset and they know that there is one disk, they might just try to
access it directly via the IDE interface?  I am thinking of some old
versions of Windows. Are they really able to cope with the case where
they can only access the disk via the legacy BIOS until non-IDE drivers
come along?


> > 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?
> 
> That sounds certainly like a better step forward than adding a crude
> hack to AHCI.
> 
> And if you want to make me completely happy, plan to extend SeaBIOS so
> that we can even drop the existing hack in IDE.

There are lots of existing guests which write to the magic ioport to
"unplug" disks.  We would need some sort of deprecation plan.