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. Alex > > Paolo -- 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
