On Wed, Mar 06, 2013 at 12:46:52PM +0100, Alexander Graf wrote: > > On 06.03.2013, at 12:44, Paolo Bonzini wrote: > > > > >>>> 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? > > > > Nothing is insurmountable. However, forcing a particular order > > of device creation is not very nice on userspace. If the hypervisor > > wants to do that, it can do userspace the favor of keeping the id > > in kernel. :) > > > >>> 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. > > > > Yes, KVM_GET_IRQCHIP_DEVICE is specific to irqchips because (following > > the model of x86) the irqchip type is chosen before creating VCPUs. > > I don't see an alternative unless we stop having irqchip as an > > all-or-nothing choice. > > > > I'm not saying KVM_CREATE_DEVICE is a bad interface, but I'm not > > sure it is really what is needed in this case. KVM_CREATE_DEVICE > > would be perfect as a replacement for KVM_CREATE_PIT2, for example. > > But in this case creating a device is not what we're really doing; > > the creation is done magically by the hypervisor by virtue of > > the previous KVM_CREATE_IRQCHIP_ARGS. > > No, it's not and it shouldn't be. To speak in x86 terms: > > KVM_SET_IRQCHIP_TYPE spawns LAPICs (indirectly, they only get spawned on vcpu creation) > KVM_CREATE_DEVICE spawns IOAPICs. > > Agree. Lumping up in-kernel LAPIC and IRQCHIPS under one in-kernel irqchip umbrella was a mistake on x86. The one we should not force on others. -- Gleb. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
