On 2011-01-20 20:37, Anthony Liguori wrote: > On 01/20/2011 03:33 AM, Jan Kiszka wrote: >> On 2011-01-19 20:32, Blue Swirl wrote: >> >>> On Wed, Jan 19, 2011 at 4:57 PM, Anthony Liguori >>> <aliguori@xxxxxxxxxxxxxxxxxx> wrote: >>> >>>> On 01/19/2011 07:15 AM, Markus Armbruster wrote: >>>> >>>>> So they interact with KVM (need kvm_state), and they interact with the >>>>> emulated PCI bus. Could you elaborate on the fundamental difference >>>>> between the two interactions that makes you choose the (hypothetical) >>>>> KVM bus over the PCI bus as device parent? >>>>> >>>>> >>>> It's almost arbitrary, but I would say it's the direction that I/Os >>>> flow. >>>> >>>> But if the underlying observation is that the device tree is not >>>> really a >>>> tree, you're 100% correct. This is part of why a factory interface >>>> that >>>> just takes a parent bus is too simplistic. >>>> >>>> I think we ought to introduce a -pci-device option that is >>>> specifically for >>>> creating PCI devices that doesn't require a parent bus argument but >>>> provides >>>> a way to specify stable addressing (for instancing, using a linear >>>> index). >>>> >>> I think kvm_state should not be a property of any device or bus. It >>> should be split to more logical pieces. >>> >>> Some parts of it could remain in CPUState, because they are associated >>> with a VCPU. >>> >>> Also, for example irqfd could be considered to be similar object to >>> char or block devices provided by QEMU to devices. Would it make sense >>> to introduce new host types for passing parts of kvm_state to devices? >>> >>> I'd also make coalesced MMIO stuff part of memory object. We are not >>> passing any state references when using cpu_physical_memory_rw(), but >>> that could be changed. >>> >> There are currently no VCPU-specific bits remaining in kvm_state. It may >> be a good idea to introduce an arch-specific kvm_state and move related >> bits over. It may also once be feasible to carve out memory management >> related fields if we have proper abstractions for that, but I'm not >> completely sure here. >> >> Anyway, all these things are secondary. The primary topic here is how to >> deal with kvm_state and its fields that have VM-global scope. >> > > The debate is really: > > 1) should we remove all passing of kvm_state and just assume it's static > > 2) deal with a couple places in the code where we need to figure out how > to get at kvm_state > > I think we've only identified 1 real instance of (2) and it's resulted > in some good discussions about how to model KVM devices vs. emulated > devices. Honestly, (1) just stinks. I see absolutely no advantage to > it at all. In the very worst case scenario, the thing we need to do is > just reference an extern variable in a few places. That completely > avoids all of the modelling discussions for now (while leaving for > placeholder FIXMEs so the problem can be tackled later). The PCI bus discussion is surely an interesting outcome, but now almost completely off-topic to the original, way less critical issue (as we were discussing internals). > > I don't understand the resistance here. IMHO, most suggestions on the table are still over-designed (like a KVMBus that only passes a kvm_state - or do you have more features for it in mind?). The idea I love most so far is establishing a machine state that also carries those few KVM bits which correspond to the KVM extension of CPUState. But in the end I want an implementable consensus that helps moving forward with main topic: the overdue KVM upstream merge. I just do not have a clear picture yet. Jan
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