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

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On 2021/12/14 17:52, Marc Zyngier wrote:
On Tue, 14 Dec 2021 08:16:40 +0000,
Qu Wenruo <quwenruo.btrfs@xxxxxxx> wrote:

On 2021/12/14 15:53, Michal Prívozník wrote:
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@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
with upstream kernels.

For RPI CM4 its mostly smooth sail, but on RK3399 due to its
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
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
mostly fail to start up due to vcpu initialization failed with

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

Not sure if it's possible to pin each vCPU thread to each core, but let
me try.

Sure it is, for instance:

      <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"/>

That's what I have already tried before.
I pinned vcpu 0-6 to physical core 0-6, and still no reliable boot up.

And that's why I'm asking here.

You are still missing the point of how QEMU works:

- QEMU creates a dummy VM with a single vcpu. This can happen on *any*

This is the main point that I missed.

Thanks very much for point this out.

- It snapshots the sysregs for this vcpu, and keep them for later
- It then destroy this VM
- QEMU then creates the full VM, with all the vcpus
- Each vcpu gets initialised with the state saved earlier. If any vcpu
   is initialised on a physical CPU of a different type from the one
   that has been used for the dummy VM, you lose, as we cannot restore
   some of the registers such as MIDR_EL1 (and other registers that KVM
   considers as invariant).

To fix this, you need to change QEMU's notion of a template VM, or
change KVM's notion of invariant registers. The former is quite hard,
and the later breaks a ton of things for guests, such as errata

The best workaround is to taskset the QEMU process (and I really mean
the process, not individual threads) to an homogeneous set of CPUs and
be done with it.

Yeah, that's why the cpuset way is working, as it seems also limiting
the initial "temporary" VM creating to specified CPUs.

Just curious, is there some defined common VM related registers that can
be restore on all cores? (At least for A53 + A72 case).

If completely no, then virtualization is really not even targeted for
BIG.little designs I guess.



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