Re: [Qemu-devel] [PATCH V12 05/17] xen: Add xenfv machine

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2011-04-13 13:49, Stefano Stabellini wrote:
> On Wed, 13 Apr 2011, Jan Kiszka wrote:
>> On 2011-04-13 12:56, Stefano Stabellini wrote:
>>> On Tue, 12 Apr 2011, Jan Kiszka wrote:
>>>> Well, either you have a use for the VCPU state (how do you do migration
>>>> in Xen without it?), or you should probably teach QEMU in a careful &
>>>> clean way to run its device model without VCPUs - and without any
>>>> TCG-related memory consumption. For the latter, you would likely receive
>>>> kudos from KVM people as well.
>>>>
>>>> BTW, if you happen to support that crazy vmport under Xen, not updating
>>>> the VCPU state will break your neck. Also, lacking VCPUs prevent the
>>>> usage of analysis and debugging features of QEMU (monitor, gdbstub).
>>>
>>> We don't use the vcpu state in qemu because qemu takes care of device
>>> emulation only; under xen the vcpu state is saved and restored by the
>>> hypervisor.
>>
>> Just out of curiosity: So you are extracting the device states out of
>> QEMU on migration, do the same with the VCPU states from the hypervisor
>> (which wouldn't be that different from KVM in fact), and then transfer
>> that to the destination node? Is there a technical or historical reason
>> for this split-up? I mean, you still need some managing instance that
>> does the state transportation and VM control on both sides, i.e. someone
>> for the job that QEMU is doing for TCG or KVM migrations.
> 
> That someone is the "toolstack", I guess libvirt would be the closest
> thing to our toolstack in the kvm world.
> The reason why we have a toolstack performing this task rather than qemu
> is that pure PV guests don't need device emulation, so we don't even
> have qemu running most of the times if there are only linux guests
> installed in the system.

Ah, for that use case it makes some sense to me.

I bet there would also be some value in consolidating the "toolstack"
functionality over bare qemu/libvirt infrastructure (if we ignored all
existing interfaces and dependencies for a moment).

Thanks,
Jan

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