Re: [Qemu-devel] [RFC V2 10/10] cpus: reclaim allocated vCPU objects

[Gu Zheng <guz.fnst@cn.fujitsu.com>]

Hi Peter,
On 12/08/2014 05:26 PM, Peter Maydell wrote:

> On 8 December 2014 at 09:16, Bharata B Rao <bharata.rao@gmail.com> wrote:
>> So this approach of parking the vcpu fd in QEMU and reusing it next
>> time during CPU hotplug will only work if all architectures supported
>> by KVM driver are ok with reuse of vcpu object in the kernel. I am
>> using relevant bits of this patch of yours with PowerPC architecture
>> to support cpu unplug and I realize that there is at least one case
>> where reuse of vcpu object in the kernel causes problems. Before I
>> attempt to fix that in ppc KVM, I wanted to check if there is
>> consensus on this approach (parking and reusing kvm vcpu fd) in QEMU.
> 
> For ARM we've just clarified the kernel-to-userspace ABI, and
> among other things it says you can't reuse the VCPU unless it's
> for exactly the same CPU type and feature configuration. So

Yes, it's a limitation now, but it is not the final implementation.
As to the QEMU side, the vcpu object in kernel is stateless, and a
reset operation will be done when we hot add a vcpu, so I think we
can kill the limitation, just choose a parked vcpu and reconfigure
it before we use it.

Thanks,
Gu 

> I'm not sure this park-and-reuse approach will work...

> 

> -- PMM
> .
>