Re: [Xen-devel] [Qemu-devel] [PATCH] Do not emulate a floppy drive when -nodefaults

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 13 May 2015, John Snow wrote:
> On 05/13/2015 02:15 PM, Stefano Stabellini wrote:
> > On Wed, 13 May 2015, Daniel P. Berrange wrote:
> >> On Wed, May 13, 2015 at 06:29:46PM +0100, Stefano Stabellini wrote:
> >>> Do not emulate a floppy drive if no drives are supposed to be present.
> >>>
> >>> This fixes the behavior of -nodefaults, that should remove the floppy
> >>> drive (see docs/qdev-device-use.txt:Default Devices), but actually
> >>> doesn't.
> >>
> >> Technically that doc is just refering to the disablement of the
> >> primary floppy drive, as opposed to the floppy controller. The
> >> floppy controller itself is really a built-in device that is
> >> defined as part of the machine type, along with the various other
> >> platform devices hanging off the PIIX and ISA brige.
> > 
> > I think you are right, this patch is a bit too harsh in fixing the
> > problem: I just wanted to properly disable drive emulation, because from
> > my tests the guest thinks that one drive is present even when is not.
> > 
> 
> We should just be a little careful in explaining the difference between
> the controller, the drives, and what circumstances things show up and to
> whom.
> 
> Currently the FDC is always present and always has two drive objects
> that are directly inlined to that device.
> 
> Now, based on whether or not this line fires in vl.c:
> default_drive(default_floppy, snapshot, IF_FLOPPY, 0, FD_OPTS);
> 
> controls whether or not we find that drive in pc_basic_device_init as
> you've found, which controls (ultimately) whether or not
> fdctrl->drives[0].blk or fdctrl->drives[1].blk gets set.
> 
> If the blk pointer is set, even if you have no media inserted,
> fd_revalidate (and pick_geometry) will *FORCIBLY PICK* a drive type for
> this floppy, currently a 1.44MB type, (20 sect, 80 track, 1 head) with a
> data rate of 500 Kbps. This is written to the rtc memory where seabios
> later reads it to discover if you have a guest-visible floppy drive there.
> 
> Both QEMU and SeaBIOS confuse the concept of "A drive is present" with
> "A disk is present" which leads to these kinds of confusing scenarios.
> 
> There's some work to do here, for sure.

That was my impression too.



On Thu, 14 May 2015, Stefan Weil wrote:
> Am 13.05.2015 um 20:15 schrieb Stefano Stabellini:
> > On Wed, 13 May 2015, Daniel P. Berrange wrote:
> > > On Wed, May 13, 2015 at 06:29:46PM +0100, Stefano Stabellini wrote:
> > > > Do not emulate a floppy drive if no drives are supposed to be present.
> > > > 
> > > > This fixes the behavior of -nodefaults, that should remove the floppy
> > > > drive (see docs/qdev-device-use.txt:Default Devices), but actually
> > > > doesn't.
> > > Technically that doc is just refering to the disablement of the
> > > primary floppy drive, as opposed to the floppy controller. The
> > > floppy controller itself is really a built-in device that is
> > > defined as part of the machine type, along with the various other
> > > platform devices hanging off the PIIX and ISA brige.
> > I think you are right, this patch is a bit too harsh in fixing the
> > problem: I just wanted to properly disable drive emulation, because from
> > my tests the guest thinks that one drive is present even when is not.
> 
> A short test on some of my physical machines shows that most
> of them don't have a floppy disk controller at all (dmesg | grep FDC).
> 
> Only some older machines still have one.
> 
> Therefore I think that QEMU must also be able to offer a virtual
> machine without an FDC, maybe as the default for the next
> version of QEMU.

I think it would make sense to make it the default for -nodefaults
machines. I would be OK with a new property too, as we could set it from
libxl or libvirt. Anybody would be happy to pick this one up or should I
do it?

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