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

To: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Subject: [Xen-devel] Re: [Qemu-devel] [PATCH V12 05/17] xen: Add xenfv machine
From: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
Date: Wed, 13 Apr 2011 13:28:07 +0200
Cc: Anthony Perard <anthony.perard@xxxxxxxxxx>, Xen Devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Alexander Graf <agraf@xxxxxxx>, QEMU-devel <qemu-devel@xxxxxxxxxx>
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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.


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