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

["Daniel P. Berrange" <berrange@xxxxxxxxxx>]

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.

> 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.

Regards,
Daniel
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