Libvirt on little.BIG ARM systems unable to start guest if no cpuset is provided

[Qu Wenruo]

On 2021/12/13 21:17, Michal Prívozník wrote:
> On 12/11/21 02:58, Qu Wenruo wrote:
>> Hi,
>>
>> Recently I got my libvirt setup on both RK3399 (RockPro64) and RPI CM4,
>> with upstream kernels.
>>
>> For RPI CM4 its mostly smooth sail, but on RK3399 due to its little.BIG
>> setup (core 0-3 are 4x A55 cores, and core 4-5 are 2x A72 cores), it
>> brings quite some troubles for VMs.
>>
>> In short, without proper cpuset to bind the VM to either all A72 cores
>> or all A55 cores, the VM will mostly fail to boot.
>>
>> Currently the working xml is:
>>
>>    <vcpu placement='static' cpuset='4-5'>2</vcpu>
>>    <cpu mode='host-passthrough' check='none'/>
>>
>> But even with vcpupin, pinning each vcpu to each physical core, VM will
>> mostly fail to start up due to vcpu initialization failed with -EINVAL.
>>
>>
>> This brings a problem, in theory RK3399 SoC should out-perform BCM2711
>> in multi-core performance, but if a VM can only be bind to either A72 or
>> A55 cores, then the performance is no longer competitive against
>> BCM2711, wasting the PCIE 2.0 x4 capacity.
>>
>> I guess with projects like Asahi Linux making progress, there will be
>> more and more such problems.
>>
>> Any clue on how to properly pass all physical CPU cores to VM for
>> little.BIG setup?
>>
>
> I have never met big.LITTLE but my understanding was that those big
> cores are compatible with little ones and the only difference is that
> the big ones are shut off if there's no demand (to save energy) leaving
> only the little ones running.

The big ones are not disabled AFAIK.

And even changing the CPU model to A53 (the little ones), it still fails
to boot, thus it looks like A72 is not really able to emulate A53 cores?

>
> Anyway, this is likely too high level forum and I'd ask QEMU developers:
>
> https://www.qemu.org/support/

That's indeed the case, adding qemu to the CC list.

And I found an existing bug report:
https://bugs.linaro.org/show_bug.cgi?id=1443

But I still didn't get the point why the 1:1 CPU-to-vcpu mapping still
doesn't work.

Thanks,
Qu

>
> Michal
>