Re: Question about migration on ARM system

[Marc Zyngier <maz@kernel.org>]

On Thu, 14 Nov 2024 09:03:24 +0000,
Zhou Wang <wangzhou1@hisilicon.com> wrote:
> 
> Hi,
> 
> I am tring to heterogeneous live migration on ARM64 host. Now I just use
> below kernel/qemu branch to have a try:
> https://github.com/hisilicon/kernel-dev/tree/private-v6.11-rc2-hisi-migrn-wip
> https://github.com/hisilicon/qemu/tree/stable-8.2-kunpeng920-initial-v2
> 
> Currently guest on GICv4.1 and GICv3 host can do migration successfully. I am
> not sure that it is expected.

It isn't. Or rather, it shouldn't. Are you reporting a KVM problem or
a QEMU problem?

> 
> GICv4.1 exports GICD_TYPER2.nASSGIcap and GICD_CTRL.nASSGIreq to guest, so
> guests(GICv3) on GICv4.1 and GICv3 host will have different state here. So
> basically a guest on host GICv4.1 will lost state when it move to a host with
> GICv3.
> 
> It seems that current mainline qemu does not consider GICD_TYPER2, and guest
> OS(linux) does not use GICD_TYPER2.nASSGIcap and GICD_CTRL.nASSGIreq(just
> check cap and set req), so current test has been passed.

Well, the guest won't test that after migration. It doesn't even know
it is migrated. But Linux definitely uses it at probe time.

> 
> In fact, there was some discussions about this problem:
> https://lore.kernel.org/kvmarm/20200304203330.4967-21-maz@kernel.org/
> 
> I am not sure if we should prevent users from doing migration between host
> GICv3 and GICv4.1?

We should allow the v3->v4.1 path (we can hide the nAS SGIs), but we
should not it the other way around.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.