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

[Michal Prívozník]

On 12/14/21 01:41, Qu Wenruo wrote:
> 
> 
> On 2021/12/14 00:49, Marc Zyngier wrote:
>> On Mon, 13 Dec 2021 16:06:14 +0000,
>> Peter Maydell <peter.maydell@linaro.org> wrote:
>>>
>>> KVM on big.little setups is a kernel-level question really; I've
>>> cc'd the kvmarm list.
>>
>> Thanks Peter for throwing us under the big-little bus! ;-)
>>
>>>
>>> On Mon, 13 Dec 2021 at 15:02, Qu Wenruo <quwenruo.btrfs@gmx.com> wrote:
>>>>
>>>>
>>>>
>>>> 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.
>>
>> s/A55/A53/. There were thankfully no A72+A55 ever produced (just the
>> though of it makes me sick).
>>
>>>>>>
>>>>>> 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.
>>
>> Disclaimer: I know nothing about libvirt (and no, I don't want to
>> know! ;-).
>>
>> However, for things to be reliable, you need to taskset the whole QEMU
>> process to the CPU type you intend to use.
> 
> Yep, that's what I'm doing.
> 
>> That's because, AFAICT,
>> QEMU will snapshot the system registers outside of the vcpu threads,
>> and attempt to use the result to configure the actual vcpu threads. If
>> they happen to run on different CPU types, the sysregs will differ in
>> incompatible ways and an error will be returned. This may or may not
>> be a bug, I don't know (I see it as a feature).
> 
> Then this brings another question.
> 
> If we can pin each vCPU to each physical core (both little and big),
> then as long as the registers are per-vCPU based, it should be able to
> pass both big and little cores to the VM.
> 
> Yeah, I totally understand this screw up the scheduling, but that's at
> least what (some insane) users want (just like me).
> 
>>
>> If you are annoyed with this behaviour, you can always use a different
>> VMM that won't care about such difference (crosvm or kvmtool, to name
>> a few).
> 
> Sounds pretty interesting, a new world but without libvirt...
> 
>> However, the guest will be able to observe the migration from
>> one cpu type to another. This may or may not affect your guest's
>> behaviour.
> 
> Not sure if it's possible to pin each vCPU thread to each core, but let
> me try.
> 

Sure it is, for instance:

<cputune>
    <vcpupin vcpu="0" cpuset="1-4,^2"/>
    <vcpupin vcpu="1" cpuset="0,1"/>
    <vcpupin vcpu="2" cpuset="2,3"/>
    <vcpupin vcpu="3" cpuset="0,4"/>
    <emulatorpin cpuset="1-3"/>
    <iothreadpin iothread="1" cpuset="5,6"/>
    <iothreadpin iothread="2" cpuset="7,8"/>
</cputune>

pins vCPU#0 onto host CPUs 1-4, excluding 2; vCPU#1 onto host CPUs 0-1
and so on. You can also pin emulator (QEMU) and its iothreads. It's
documented here:

https://libvirt.org/formatdomain.html#cpu-tuning

Michal