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

[Marcel Apfelbaum]

On 03/23/2015 10:11 PM, Paolo Bonzini wrote:
> 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 <address@hidden>
> > > > ---
> > > >    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.
Personally I prefer this one, but out of the scope of this patch.

> > > 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.
You got me there :)
From what I understood for those boards there is no need for this
combination but I don't know them enough (OK.. at all).

> > 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.
Always migration wins.

Bottom line, of course I don't have anything against fixing this bug,
my problem was only with the way we add those fields (usb_disabled), maybe a 
three state
QOM property (and variable behind it) is a solution, but not for now of course.

I also didn't like the required/allowed fields and I added them anyway...

Thanks,
Marcel


> Paolo