Question about KVM on Big.Little SoC

Question about KVM on Big.Little SoC

[Peng Fan]

Hi Andre,

I came across your patch which try to fix kvm on big.little.
https://lists.cs.columbia.edu/pipermail/kvmarm/2013-April/003989.html

I am trying KVM on a big.little SoC with 4 A53 and 2 A72.
I wonder whether kvm supports running on big.little SoC now?

I am checking arch/{arm,arm64}/kvm code, but I think you may give a quick answer.

And If kvm supports running on big.Little SoC, is it ok for me to pass "-cpu host"
to qemu? Can guest vcpu be scheduled freely on big and little SoCs?

Would you please kindly give me some clues about where is the piece code to handle
kvm on big.little?

Thanks,
Peng.

Re: Question about KVM on Big.Little SoC

[Marc Zyngier]

Peng,

On 06/09/16 09:00, Peng Fan wrote:
> Hi Andre,
> 
> I came across your patch which try to fix kvm on big.little.
> https://lists.cs.columbia.edu/pipermail/kvmarm/2013-April/003989.html
> 
> I am trying KVM on a big.little SoC with 4 A53 and 2 A72.
> I wonder whether kvm supports running on big.little SoC now?
> 
> I am checking arch/{arm,arm64}/kvm code, but I think you may give a quick answer.
> 
> And If kvm supports running on big.Little SoC, is it ok for me to pass "-cpu host"
> to qemu? Can guest vcpu be scheduled freely on big and little SoCs?
> 
> Would you please kindly give me some clues about where is the piece code to handle
> kvm on big.little?

The current situation is that:

- Yes, KVM works on big-little systems (it always had)
- The only useful option you can pass to qemu is "-cpu host"
- There is no guarantee that QEMU will even start, unless you constraint
its affinity to one single type of CPUs (use taskset).
- If you use taskset, there will be no scheduling between types of cores
- If you do not, scheduling will happen, but that's a fairly unsafe
thing to do (the guest doesn't know it is being migrated between CPUs
that may have different behaviours...).

Hope this helps,

	M.
-- 
Jazz is not dead. It just smells funny...

Re: Question about KVM on Big.Little SoC

[Peng Fan]

Hi Marc,
On Tue, Sep 06, 2016 at 09:12:26AM +0100, Marc Zyngier wrote:
>Peng,
>
>On 06/09/16 09:00, Peng Fan wrote:
>> Hi Andre,
>> 
>> I came across your patch which try to fix kvm on big.little.
>> https://lists.cs.columbia.edu/pipermail/kvmarm/2013-April/003989.html
>> 
>> I am trying KVM on a big.little SoC with 4 A53 and 2 A72.
>> I wonder whether kvm supports running on big.little SoC now?
>> 
>> I am checking arch/{arm,arm64}/kvm code, but I think you may give a quick answer.
>> 
>> And If kvm supports running on big.Little SoC, is it ok for me to pass "-cpu host"
>> to qemu? Can guest vcpu be scheduled freely on big and little SoCs?
>> 
>> Would you please kindly give me some clues about where is the piece code to handle
>> kvm on big.little?
>
>The current situation is that:
>
>- Yes, KVM works on big-little systems (it always had)
>- The only useful option you can pass to qemu is "-cpu host"
>- There is no guarantee that QEMU will even start, unless you constraint
>its affinity to one single type of CPUs (use taskset).
>- If you use taskset, there will be no scheduling between types of cores
>- If you do not, scheduling will happen, but that's a fairly unsafe
>thing to do (the guest doesn't know it is being migrated between CPUs
>that may have different behaviours...).

Thanks for your explaination.
I think I should use 'taskset [cpumask] qemu-system-aarch64 ....' to start
guest os -:)

>
>Hope this helps,

Yeah. This helps. I am trying using taskset.

Thanks,
Peng.

>
>	M.
>-- 
>Jazz is not dead. It just smells funny...