linux-kernel - Re: [RFC PATCH v2 3/4] KVM: arm64: GICv4.1: Restore VLPI's pending state to physical side

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Date:   Tue, 05 Jan 2021 09:25:11 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Shenming Lu <lushenming@...wei.com>
Cc:     Eric Auger <eric.auger@...hat.com>, Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Alex Williamson <alex.williamson@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        wanghaibin.wang@...wei.com, yuzenghui@...wei.com
Subject: Re: [RFC PATCH v2 3/4] KVM: arm64: GICv4.1: Restore VLPI's pending
 state to physical side

On 2021-01-04 08:16, Shenming Lu wrote:
> From: Zenghui Yu <yuzenghui@...wei.com>
> 
> When setting the forwarding path of a VLPI (switch to the HW mode),
> we could 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@...wei.com>
> Signed-off-by: Shenming Lu <lushenming@...wei.com>
> ---
>  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 f211a7c32704..7945d6d09cdd 100644
> --- a/arch/arm64/kvm/vgic/vgic-v4.c
> +++ b/arch/arm64/kvm/vgic/vgic-v4.c
> @@ -454,6 +454,18 @@ 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);

Why do this if pending_latch is 0, which is likely to be
the overwhelming case?

> +
> +	/*
> +	 * Let it be pruned from ap_list later and don't bother
> +	 * the List Register.
> +	 */
> +	irq->pending_latch = false;

What guarantees the pruning? Pruning only happens on vcpu exit,
which means we may have the same interrupt via both the LR and
the stream interface, which I don't believe is legal (it is
like having two LRs holding the same interrupt).

> +
>  out:
>  	mutex_unlock(&its->its_lock);
>  	return ret;

Thanks,

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