On Wed, Mar 06, 2013 at 11:04:21AM +0100, Alexander Graf wrote:
>
>
> Am 06.03.2013 um 10:58 schrieb Gleb Natapov <[email protected]>:
>
> > On Wed, Mar 06, 2013 at 10:40:18AM +0100, Paolo Bonzini wrote:
> >> Il 05/03/2013 16:25, Gleb Natapov ha scritto:
> >>>> 1) We need to set the generic interrupt type of the system before we
> >>>> create vcpus.
> >>>>
> >>>> This is a new ioctl that sets the overall system interrupt controller
> >>>> type to a specific model. This used so that when we create vcpus, we can
> >>>> create the appended "local interrupt controller" state without the
> >>>> actual interrupt controller device available yet. It is also used later
> >>>> to switch between interrupt controller implementations.
> >>>>
> >>>> This interrupt type is write once and frozen after the first vcpu got
> >>>> created.
> >>>
> >>> Why explicit ioctl is needed? Why not require specific irqchip to be
> >>> created before first vcpu. The device created determines system interrupt
> >>> controller type.
> >>
> >> QEMU creates CPUs before devices, and CPUs need to know what kind of
> >> local interrupt controller to create. Similar to how in-kernel LAPIC
> >> state is created long before the userspace device that proxies the LAPIC.
> > So what is the difference between calling this special ioctl before
> > creating vcpus and calling create device ioctl instead and create QEMU
> > proxy device at whatever point in time QEMU wants to create it?
>
> I don't understand the question really. What proxy device?
>
That's how Paolo called the QEMU part of the kernel irqchip device.
So the question is this. You propose special ioctl to set "irqchip
architecture". Lets call it SET_IRQCHIP_ARCH. QEMU suppose to do that:
ioctl(SET_IRQCHIP_ARCH, MPIC)
create_vcpus()
create_devices()
create_irqchips()
ioctl(CREATE_DEVICE, MPIC)
ioctl(SET_ATTR, attr1)
ioctl(SET_ATTR, attr2)
Why can't it do:
ioctl(CREATE_DEVICE, MPIC)
create_vcpus()
create_irqchips()
ioctl(SET_ATTR, attr1)
ioctl(SET_ATTR, attr2)
The question is rhetorical though because I know it can, it does that
for x86. So the real question is what is the disadvantages that warrant
separate ioctl?
--
Gleb.
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