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

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On 2021/12/14 18:36, Daniel P. Berrangé wrote:
On Tue, Dec 14, 2021 at 09:34:18AM +0000, Marc Zyngier wrote:
On Tue, 14 Dec 2021 00:41:01 +0000,
Qu Wenruo <quwenruo.btrfs@xxxxxxx> wrote:

On 2021/12/14 00:49, Marc Zyngier wrote:
On Mon, 13 Dec 2021 16:06:14 +0000,
Peter Maydell <peter.maydell@xxxxxxxxxx> 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@xxxxxxx> wrote:

On 2021/12/13 21:17, Michal Prívozník wrote:
On 12/11/21 02:58, Qu Wenruo wrote:

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.

Are you sure? The xml directive above seem to only apply to the vcpus,
and no other QEMU thread.

For historical reasons this XML element is a bit misleadingly named.

With the config

    <vcpu placement='static' cpuset='4-5'>2</vcpu>

the 'cpuset' applies to the QEMU process as a whole - its vCPUs,
I/O threads and any other emulator threads.

There is a separate config for setting per-VCPU binding which was
illustrated elsewhere in this thread.

Which also means, I can put the io threads to A53 cores freeing up the
A72 cores more.

And is there any plan to deprecate the old "cpuset" key of vcpu element,
and recommend to use "vcpupin" element?



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