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Re: [PATCH for-6.0 2/8] spapr/xive: Introduce spapr_xive_nr_ends()

From: Greg Kurz
Subject: Re: [PATCH for-6.0 2/8] spapr/xive: Introduce spapr_xive_nr_ends()
Date: Wed, 25 Nov 2020 13:26:03 +0100

On Wed, 25 Nov 2020 12:34:25 +0100
Cédric Le Goater <clg@kaod.org> wrote:

> >>> This complexifies migration because we have to guess at
> >>> post load if we should claim the IPI in KVM or not. The
> >>> simple presence of the vCPU isn't enough : we need to
> >>> guess if the guest actually configured the IPI or not.
> >>
> >> The EAT will be transferred from the source and the call to 
> >> kvmppc_xive_source_reset_one() should initialize the KVM 
> >> device correctly on the target for all interrupts.
> >>
> > 
> > Except that the EAS appears as valid for all IPIs, even
> > though the source didn't claim them at the KVM level. 
> why ? we would stop claiming IPIs in spapr_irq_init() and so
> they won't appear as being valid anymore, at boot time or
> restore time.

If we don't claim the IPIs in spapr_irq_init() anymore then
we must at least claim them on the path of H_INT_GET_SOURCE_INFO
otherwise it will fail with H_P2 and the guest won't even
try to setup the IPI. Even if we do that, we still have a
window where the source is valid in QEMU but not yet at
the KVM level.

> C.

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