Re: [PATCH v3 6/7] iommu/mediatek: Gather iova in iommu_unmap to achieve tlb sync once

[Tomasz Figa <tfiga@chromium.org>]

On Wed, Dec 16, 2020 at 06:36:06PM +0800, Yong Wu wrote:
> In current iommu_unmap, this code is:
> 
> 	iommu_iotlb_gather_init(&iotlb_gather);
> 	ret = __iommu_unmap(domain, iova, size, &iotlb_gather);
> 	iommu_iotlb_sync(domain, &iotlb_gather);
> 
> We could gather the whole iova range in __iommu_unmap, and then do tlb
> synchronization in the iommu_iotlb_sync.
> 
> This patch implement this, Gather the range in mtk_iommu_unmap.
> then iommu_iotlb_sync call tlb synchronization for the gathered iova range.
> we don't call iommu_iotlb_gather_add_page since our tlb synchronization
> could be regardless of granule size.
> 
> In this way, gather->start is impossible ULONG_MAX, remove the checking.
> 
> This patch aims to do tlb synchronization *once* in the iommu_unmap.
> 
> Signed-off-by: Yong Wu <yong.wu@mediatek.com>
> ---
>  drivers/iommu/mtk_iommu.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/iommu/mtk_iommu.c b/drivers/iommu/mtk_iommu.c
> index db7d43adb06b..89cec51405cd 100644
> --- a/drivers/iommu/mtk_iommu.c
> +++ b/drivers/iommu/mtk_iommu.c
> @@ -506,7 +506,12 @@ static size_t mtk_iommu_unmap(struct iommu_domain *domain,
>  			      struct iommu_iotlb_gather *gather)
>  {
>  	struct mtk_iommu_domain *dom = to_mtk_domain(domain);
> +	unsigned long long end = iova + size;
>  
> +	if (gather->start > iova)
> +		gather->start = iova;
> +	if (gather->end < end)
> +		gather->end = end;

I don't know how common the case is, but what happens if
gather->start...gather->end is a disjoint range from iova...end? E.g.

 | gather      | ..XXX... | iova |
 |             |          |      |
 gather->start |          iova   |
               gather->end       end

We would also end up invalidating the TLB for the XXX area, which could
affect the performance.

Also, why is the existing code in __arm_v7s_unmap() not enough? It seems
to call io_pgtable_tlb_add_page() already, so it should be batching the
flushes.

>  	return dom->iop->unmap(dom->iop, iova, size, gather);
>  }
>  
> @@ -523,9 +528,6 @@ static void mtk_iommu_iotlb_sync(struct iommu_domain *domain,
>  	struct mtk_iommu_domain *dom = to_mtk_domain(domain);
>  	size_t length = gather->end - gather->start;
>  
> -	if (gather->start == ULONG_MAX)
> -		return;
> -
>  	mtk_iommu_tlb_flush_range_sync(gather->start, length, gather->pgsize,
>  				       dom->data);
>  }
> -- 
> 2.18.0
> 
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