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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

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.


> > diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> > index a8e6be1..c9f50b3 100644
> > --- a/hw/i386/pc.c
> > +++ b/hw/i386/pc.c
> > @@ -1410,6 +1410,7 @@ void pc_basic_device_init(ISABus *isa_bus, qemu_irq 
> > *gsi,
> >      qemu_irq *cpu_exit_irq;
> >      MemoryRegion *ioport80_io = g_new(MemoryRegion, 1);
> >      MemoryRegion *ioportF0_io = g_new(MemoryRegion, 1);
> > +    bool floppy_exist;
> >  
> >      memory_region_init_io(ioport80_io, NULL, &ioport80_io_ops, NULL, 
> > "ioport80", 1);
> >      memory_region_add_subregion(isa_bus->address_space_io, 0x80, 
> > ioport80_io);
> > @@ -1487,10 +1488,17 @@ void pc_basic_device_init(ISABus *isa_bus, qemu_irq 
> > *gsi,
> >      cpu_exit_irq = qemu_allocate_irqs(cpu_request_exit, NULL, 1);
> >      DMA_init(0, cpu_exit_irq);
> >  
> > +    *floppy = NULL;
> > +    floppy_exist = false;
> >      for(i = 0; i < MAX_FD; i++) {
> >          fd[i] = drive_get(IF_FLOPPY, 0, i);
> > +        if (fd[i] != NULL) {
> > +            floppy_exist = true;
> > +        }
> > +    }
> > +    if (floppy_exist) {
> > +        *floppy = fdctrl_init_isa(isa_bus, fd);
> >      }
> > -    *floppy = fdctrl_init_isa(isa_bus, fd);
> >  }
> 
> This results in a guest ABI change when updating QEMU, because the floppy
> controller will disappear for existing guests. This is liable to break
> guests upon migration.
> 
> If we want to be able to disable the floppy controller itself, in addition
> to the floppy drives, then we'd likely need to define a property against
> the machine type to allow its existence to be toggled on/off. We'd then
> need to at least make sure the existing machine types defaulted to on,
> if we decided that new machine types should default to off.
> 
> Libvirt would also need to gain the ability to represent the existance
> of the floppy controller and allow it to be turned on / off explicitly.


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