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


From: Gu Zheng
Subject: Re: [Qemu-devel] [RFC V2 10/10] cpus: reclaim allocated vCPU objects
Date: Mon, 8 Dec 2014 18:28:49 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:7.0.1) Gecko/20110930 Thunderbird/7.0.1

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

> On 8 December 2014 at 09:16, Bharata B Rao <address@hidden> 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
> .
> 





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