Re: in-kernel interrupt controller steering

[Gleb Natapov <gleb@redhat.com>]

On Wed, Mar 06, 2013 at 05:38:33AM -0500, Paolo Bonzini wrote:
> 
> 
> ----- Messaggio originale -----
> > Da: "Gleb Natapov" <gleb@redhat.com>
> > A: "Paolo Bonzini" <pbonzini@redhat.com>
> > Cc: "Alexander Graf" <agraf@suse.de>, kvm@vger.kernel.org, kvm-ppc@vger.kernel.org, "Stuart Yoder"
> > <stuart.yoder@freescale.com>, "Scott Wood" <scottwood@freescale.com>, "Paul Mackerras" <paulus@samba.org>, "Peter
> > Maydell" <peter.maydell@linaro.org>
> > Inviato: Mercoledì, 6 marzo 2013 10:58:35
> > Oggetto: Re: in-kernel interrupt controller steering
> > 
> > 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?
> 
> Because you'd have to stash the handle that KVM_CREATE_DEVICE returns
> somewhere, waiting for the QEMU device to be created.
> 
OK, we try not to add interfaces for one userspace convenience though.
Is this such insurmountable problem for QEMU?

> Perhaps it's just a problem of naming, and KVM_CREATE_DEVICE is simply
> not the right name for the interface.  Once both KVM_CREATE_IRQCHIP_ARGS
> and KVM_CREATE_DEVICE are added, it really will not create the device anymore.
> Devices will be created by KVM_CREATE_IRQCHIP_ARGS, and possibly by
> KVM_CREATE_VCPU.  KVM_CREATE_DEVICE is really only returning an id.
> 
> So we can have this instead:
> - KVM_CREATE_IRQCHIP_ARGS becomes KVM_SET_IRQCHIP_TYPE (and "none"
> can be a valid irqchip type).
> 
> - KVM_CREATE_DEVICE becomes KVM_GET_IRQCHIP_DEVICE, and you pass it a
> device type and possibly a VCPU number.
> 
> It's mostly about names, but one important property is that
> KVM_GET_IRQCHIP_DEVICE can be called at any time and, in fact,
> multiple times.  Gleb, do you like this more?
> 
If you put it like this it sounds better (well you've just stashed the
handle in kernel for QEMU convenience :)), but you've made the interface
irqchips specific again and this is what we are trying to avoid.

--
			Gleb.