Re: [Qemu-devel] [PATCH for-2.3] powerpc: fix -machine usb=no for newworld and pseries machines

[Paolo Bonzini <pbonzini@redhat.com>]

On 23/03/2015 19:21, Alexander Graf wrote:
> On 03/23/2015 07:20 PM, Marcel Apfelbaum wrote:
>> On 03/23/2015 07:05 PM, Paolo Bonzini wrote:
>>> Capture the explicit setting of "usb=no" into a separate bool, and
>>> use it to skip the update of machine->usb in the board init function.
>>>
>>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>>> ---
>>>   hw/core/machine.c     | 1 +
>>>   hw/ppc/mac_newworld.c | 2 +-
>>>   hw/ppc/spapr.c        | 2 +-
>>>   include/hw/boards.h   | 1 +
>>>   4 files changed, 4 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/hw/core/machine.c b/hw/core/machine.c
>>> index cb1185a..25c45e6 100644
>>> --- a/hw/core/machine.c
>>> +++ b/hw/core/machine.c
>>> @@ -223,6 +223,7 @@ static void machine_set_usb(Object *obj, bool
>>> value, Error **errp)
>>>       MachineState *ms = MACHINE(obj);
>>>
>>>       ms->usb = value;
>>> +    ms->usb_disabled = !value;
>> Maybe is too late now, but I really not like this pollution of
>> MachineState
>> with 'usb_disabled'. (Imagine we have this kind of fields for lots of
>> objects and lots
>> of corner cases...)
>> I know it comes to solve a bug, but we talked about it in another mail
>> thread and
>> this change in semantics was approved.
>>
>> Let me explain *why* I don't like it.
>> 1. We add an "usb_disabled" field to a base class (actually object)
>>    of all the machines and the only place it is interesting is
>>    for 2 machines on ppc.

So we do for kernel_irqchip_requested/allowed.  Both approaches could be
replaced by a tri-state on/off/auto.

>> 2. Even for these 2 machines, the scenario of defaults=on and usb=off
>>    is not practical.

Why?  For example you could add a virtio-input device instead of a USB
keyboard and mouse.

> I'm personally fine either way, but I assumed that Paolo had a good
> reason for writing the patch?

Actually, two.

One good reason is that no matter how you look at it, it's at least
surprising and at worst a bug that "-machine usb=no" includes a default
USB controller.

The second good reason is that it's a guest ABI change for the versioned
pSeries machines, and as such it breaks migration.

Paolo