Re: [PATCH v4 5/7] iommu/dma: Allow a single FQ in addition to per-CPU FQs

[Niklas Schnelle]

On Wed, 2023-01-04 at 13:05 +0100, Niklas Schnelle wrote:
> In some virtualized environments, including s390 paged memory guests,
> IOTLB flushes are used to update IOMMU shadow tables. Due to this, they
> are much more expensive than in typical bare metal environments or
> non-paged s390 guests. In addition they may parallelize more poorly in
> virtualized environments. This changes the trade off for flushing IOVAs
> such that minimizing the number of IOTLB flushes trumps any benefit of
> cheaper queuing operations or increased paralellism.
> 
> In this scenario per-CPU flush queues pose several problems. Firstly
> per-CPU memory is often quite limited prohibiting larger queues.
> Secondly collecting IOVAs per-CPU but flushing via a global timeout
> reduces the number of IOVAs flushed for each timeout especially on s390
> where PCI interrupts may not be bound to a specific CPU.
> 
> Thus let's introduce a single flush queue mode IOMMU_DOMAIN_DMA_SQ that
> reuses the same queue logic but only allocates a single global queue
> allowing larger batches of IOVAs to be freed at once and with larger
> timeouts. This is to allow the common IOVA flushing code to more closely
> resemble the global flush behavior used on s390's previous internal DMA
> API implementation.
> 
> As we now support two different variants of flush queues rename the
> existing __IOMMU_DOMAIN_DMA_FQ to __IOMMU_DOMAIN_DMA_LAZY to indicate
> the general case of having a flush queue and introduce separate
> __IOMMU_DOMAIN_DMA_PERCPU_Q and __IOMMU_DOMAIN_DMA_SINGLE_Q bits to
> indicate the two queue variants.
> 
> Link: https://lore.kernel.org/linux-iommu/3e402947-61f9-b7e8-1414-fde006257b6f@xxxxxxx/
> Signed-off-by: Niklas Schnelle <schnelle@xxxxxxxxxxxxx>
> ---
> v2 -> v3:
> - Rename __IOMMU_DOMAIN_DMA_FQ to __IOMMU_DOMAIN_DMA_LAZY to make it more clear
>   that this bit indicates flush queue use independent of the exact queuing
>   strategy
> 
---8<---
>  
>  /* sysfs updates are serialised by the mutex of the group owning @domain */
>  int iommu_dma_init_fq(struct iommu_domain *domain)
>  {
>  	struct iommu_dma_cookie *cookie = domain->iova_cookie;
> -	struct iova_fq __percpu *queue;
> -	int i, cpu;
> +	int rc;
>  
>  	if (cookie->fq_domain)
>  		return 0;
> @@ -250,26 +336,16 @@ int iommu_dma_init_fq(struct iommu_domain *domain)
>  	atomic64_set(&cookie->fq_flush_start_cnt,  0);
>  	atomic64_set(&cookie->fq_flush_finish_cnt, 0);
>  
> 
---8<---
> +	if (domain->type == IOMMU_DOMAIN_DMA_FQ)
> +		rc = iommu_dma_init_fq_percpu(cookie);
> +	else
> +		rc = iommu_dma_init_fq_single(cookie);

Found out in testing that the above doesn't work for the "echo XYZ >
/sys/../iommu_group/type" interface as domain->type is not set before
calling iommu_dma_init_fq() so it would always init for DMA-SQ which is
of course the case that I used during earlier testing. I think the
easiest fix is to add a type parameter to iommu_dma_init_fq().

>  
> -		for (i = 0; i < IOVA_FQ_SIZE; i++)
> -			INIT_LIST_HEAD(&fq->entries[i].freelist);
> +	if (rc) {
> +		pr_warn("iova flush queue initialization failed\n");
> +		return rc;
>  	}
> 
---8<--- 
> 
>  	mutex_unlock(&group->mutex);
> @@ -2896,10 +2900,10 @@ static int iommu_change_dev_def_domain(struct iommu_group *group,
>  	}
>  
>  	/* We can bring up a flush queue without tearing down the domain */
> -	if (type == IOMMU_DOMAIN_DMA_FQ && prev_dom->type == IOMMU_DOMAIN_DMA) {
> +	if (!!(type & __IOMMU_DOMAIN_DMA_LAZY) && prev_dom->type == IOMMU_DOMAIN_DMA) {
>  		ret = iommu_dma_init_fq(prev_dom);
>  		if (!ret)
> -			prev_dom->type = IOMMU_DOMAIN_DMA_FQ;
> +			prev_dom->type = type;

Here domain->type is set only after calling iommu_dma_init_fq().

>  		goto out;
>  	}
>