Re: [PATCH 3/8] KVM: arm/arm64: vgic-its: Cache successful MSI->LPI translation

[Julien Thierry <julien.thierry@arm.com>]

Hi Marc,

On 06/06/2019 17:54, Marc Zyngier wrote:
> On a successful translation, preserve the parameters in the LPI
> translation cache. Each translation is reusing the last slot
> in the list, naturally evincting the least recently used entry.
> 
> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> ---
>  virt/kvm/arm/vgic/vgic-its.c | 41 ++++++++++++++++++++++++++++++++++++
>  1 file changed, 41 insertions(+)
> 
> diff --git a/virt/kvm/arm/vgic/vgic-its.c b/virt/kvm/arm/vgic/vgic-its.c
> index 5758504fd934..bc370b6c5afa 100644
> --- a/virt/kvm/arm/vgic/vgic-its.c
> +++ b/virt/kvm/arm/vgic/vgic-its.c
> @@ -538,6 +538,45 @@ static unsigned long vgic_mmio_read_its_idregs(struct kvm *kvm,
>  	return 0;
>  }
>  
> +static void vgic_its_cache_translation(struct kvm *kvm, struct vgic_its *its,
> +				       u32 devid, u32 eventid,
> +				       struct vgic_irq *irq)
> +{
> +	struct vgic_dist *dist = &kvm->arch.vgic;
> +	struct vgic_translation_cache_entry *cte;
> +	unsigned long flags;
> +
> +	/* Do not cache a directly injected interrupt */
> +	if (irq->hw)
> +		return;
> +
> +	raw_spin_lock_irqsave(&dist->lpi_list_lock, flags);
> +
> +	/* Always reuse the last entry (LRU policy) */
> +	cte = list_last_entry(&dist->lpi_translation_cache,
> +			      typeof(*cte), entry);
> +
> +	/*
> +	 * Caching the translation implies having an extra reference
> +	 * to the interrupt, so drop the potential reference on what
> +	 * was in the cache, and increment it on the new interrupt.
> +	 */
> +	if (cte->irq)
> +		__vgic_put_lpi_locked(kvm, cte->irq);
> +
> +	vgic_get_irq_kref(irq);

If cte->irq == irq, can we avoid the ref putting and getting and just
move the list entry (and update cte)?

Cheers,

-- 
Julien Thierry
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