Re: [Qemu-devel] [PATCH 28/35] kvm: x86: Introduce kvmclock device to save/restore its state

[Alex Williamson]

On Tue, 2011-01-18 at 16:54 +0100, Jan Kiszka wrote:
> On 2011-01-18 16:48, Anthony Liguori wrote:
> > On 01/18/2011 09:43 AM, Jan Kiszka wrote:
> >> On 2011-01-18 16:04, Anthony Liguori wrote:
> >>    
> >>> On 01/18/2011 08:28 AM, Jan Kiszka wrote:
> >>>      
> >>>> On 2011-01-12 11:31, Jan Kiszka wrote:
> >>>>
> >>>>        
> >>>>> Am 12.01.2011 11:22, Avi Kivity wrote:
> >>>>>
> >>>>>          
> >>>>>> On 01/11/2011 03:54 PM, Anthony Liguori wrote:
> >>>>>>
> >>>>>>            
> >>>>>>> Right, we should introduce a KVMBus that KVM devices are created on.
> >>>>>>> The devices can get at KVMState through the BusState.
> >>>>>>>
> >>>>>>>              
> >>>>>> There is no kvm bus in a PC (I looked).  We're bending the device model
> >>>>>> here because a device is implemented in the kernel and not in
> >>>>>> userspace.  An implementation detail is magnified beyond all 
> >>>>>> proportions.
> >>>>>>
> >>>>>> An ioapic that is implemented by kvm lives in exactly the same place
> >>>>>> that the qemu ioapic lives in.  An assigned pci device lives on the PCI
> >>>>>> bus, not a KVMBus.  If we need a pointer to KVMState, then we must find
> >>>>>> it elsewhere, not through creating imaginary buses that don't exist.
> >>>>>>
> >>>>>>
> >>>>>>            
> >>>>> Exactly.
> >>>>>
> >>>>> So we can either "infect" the whole device tree with kvm (or maybe a
> >>>>> more generic accelerator structure that also deals with Xen) or we need
> >>>>> to pull the reference inside the device's init function from some global
> >>>>> service (kvm_get_state).
> >>>>>
> >>>>>          
> >>>> Note that this topic is still waiting for good suggestions, specifically
> >>>> from those who believe in kvm_state references :). This is not only
> >>>> blocking kvmstate merge but will affect KVM irqchips as well.
> >>>>
> >>>> It boils down to how we reasonably pass a kvm_state reference from
> >>>> machine init code to a sysbus device. I'm probably biased, but I don't
> >>>> see any way that does not work against the idea of confining access to
> >>>> kvm_state or breaks device instantiation from the command line or a
> >>>> config file.
> >>>>
> >>>>        
> >>> A KVM device should sit on a KVM specific bus that hangs off of sysbus.
> >>> It can get to kvm_state through that bus.
> >>>
> >>> That bus doesn't get instantiated through qdev so requiring a pointer
> >>> argument should not be an issue.
> >>>
> >>>      
> >> This design is in conflict with the requirement to attach KVM-assisted
> >> devices also to their home bus, e.g. an assigned PCI device to the PCI
> >> bus. We don't support multi-homed qdev devices.
> >>    
> > 
> > With vfio, would an assigned PCI device even need kvm_state?
> 
> IIUC: Yes, for establishing the irqfd link.

We abstract this through the msi/msix layer though, so the vfio driver
doesn't directly know anything about kvm_state.

Alex