Re: [RFC PATCH v1 3/4] KVM: arm64: GICv4.1: Restore VLPI's pending state to physical side

[Marc Zyngier]

On 2020-11-24 08:10, Shenming Lu wrote:
> On 2020/11/23 17:27, Marc Zyngier wrote:
> > On 2020-11-23 06:54, Shenming Lu wrote:
> > > From: Zenghui Yu <yuzenghui@xxxxxxxxxx>
> > >
> > > When setting the forwarding path of a VLPI, it is more consistent to
> >
> > I'm not sure it is more consistent. It is a *new* behaviour, because 
> > it only
> > matters for migration, which has been so far unsupported.
>
> Alright, consistent may not be accurate...
> But I have doubt that whether there is really no need to transfer the
> pending states
> from kvm'vgic to VPT in set_forwarding regardless of migration, and the 
> similar
> for unset_forwarding.

If you have to transfer that state outside of the a save/restore, it 
means that
you have missed the programming of the PCI endpoint. This is an 
established
restriction that the MSI programming must occur *after* the translation 
has
been established using MAPI/MAPTI (see the large comment at the 
beginning of
vgic-v4.c).

If you want to revisit this, fair enough. But you will need a lot more 
than
just opportunistically transfer the pending state.

> > > also transfer the pending state from irq->pending_latch to VPT 
> > > (especially
> > > in migration, the pending states of VLPIs are restored into kvm’s 
> > > vgic
> > > first). And we currently send "INT+VSYNC" to trigger a VLPI to 
> > > pending.
> > >
> > > Signed-off-by: Zenghui Yu <yuzenghui@xxxxxxxxxx>
> > > Signed-off-by: Shenming Lu <lushenming@xxxxxxxxxx>
> > > ---
> > >  arch/arm64/kvm/vgic/vgic-v4.c | 12 ++++++++++++
> > >  1 file changed, 12 insertions(+)
> > >
> > > diff --git a/arch/arm64/kvm/vgic/vgic-v4.c 
> > > b/arch/arm64/kvm/vgic/vgic-v4.c
> > > index b5fa73c9fd35..cc3ab9cea182 100644
> > > --- a/arch/arm64/kvm/vgic/vgic-v4.c
> > > +++ b/arch/arm64/kvm/vgic/vgic-v4.c
> > > @@ -418,6 +418,18 @@ int kvm_vgic_v4_set_forwarding(struct kvm *kvm, 
> > > int virq,
> > >      irq->host_irq    = virq;
> > >      atomic_inc(&map.vpe->vlpi_count);
> > >
> > > +    /* Transfer pending state */
> > > +    ret = irq_set_irqchip_state(irq->host_irq,
> > > +                    IRQCHIP_STATE_PENDING,
> > > +                    irq->pending_latch);
> > > +    WARN_RATELIMIT(ret, "IRQ %d", irq->host_irq);
> > > +
> > > +    /*
> > > +     * Let it be pruned from ap_list later and don't bother
> > > +     * the List Register.
> > > +     */
> > > +    irq->pending_latch = false;
> >
> > It occurs to me that calling into irq_set_irqchip_state() for a large
> > number of interrupts can take a significant amount of time. It is also
> > odd that you dump the VPT with the VPE unmapped, but rely on the VPE
> > being mapped for the opposite operation.
> >
> > Shouldn't these be symmetric, all performed while the VPE is unmapped?
> > It would also save a lot of ITS traffic.
>
> My thought was to use the existing interface directly without 
> unmapping...
>
> If you want to unmap the vPE and poke the VPT here, as I said in the 
> cover
> letter, set/unset_forwarding might also be called when all devices are 
> running
> at normal run time, in which case the unmapping of the vPE is not 
> allowed...

No, I'm suggesting that you don't do anything here, but instead as a 
by-product
of restoring the ITS tables. What goes wrong if you use the
KVM_DEV_ARM_ITS_RESTORE_TABLE backend instead?

> Another possible solution is to add a new dedicated interface to QEMU
> to transfer
> these pending states to HW in GIC VM state change handler corresponding 
> to
> save_pending_tables?

Userspace has no way to know we use GICv4, and I intend to keep it
completely out of the loop. The API is already pretty tortuous, and
I really don't want to add any extra complexity to it.

Thanks,

        M.
--
Jazz is not dead. It just smells funny...