linux-kernel - Re: [PATCH v3 3/5] dma-iommu: add SKIP_CPU

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Date:   Wed, 11 Aug 2021 20:00:35 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     David Stevens <stevensd@...omium.org>,
        Will Deacon <will@...nel.org>
Cc:     Joerg Roedel <joro@...tes.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Tom Murphy <murphyt7@....ie>, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/5] dma-iommu: add SKIP_CPU_SYNC after syncing

On 2021-08-11 03:42, David Stevens wrote:
> From: David Stevens <stevensd@...omium.org>
> 
> After syncing in map/unmap, add the DMA_ATTR_SKIP_CPU_SYNC flag so
> anything that uses attrs later on will skip any sync work that has
> already been completed. In particular, this skips copying from the
> swiotlb twice during unmap.
> 
> Signed-off-by: David Stevens <stevensd@...omium.org>
> ---
>   drivers/iommu/dma-iommu.c | 13 ++++++++++---
>   1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index 4f0cc4a0a61f..be0214b1455c 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -859,8 +859,11 @@ static dma_addr_t iommu_dma_map_page(struct device *dev, struct page *page,
>   static void iommu_dma_unmap_page(struct device *dev, dma_addr_t dma_handle,
>   		size_t size, enum dma_data_direction dir, unsigned long attrs)
>   {
> -	if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC))
> +	if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC)) {
>   		iommu_dma_sync_single_for_cpu(dev, dma_handle, size, dir);
> +		attrs |= DMA_ATTR_SKIP_CPU_SYNC;
> +	}
> +
>   	__iommu_dma_unmap_swiotlb(dev, dma_handle, size, dir, attrs);

Again, fold that into here and put the arch sync in an else case to the 
is_swiotlb_buffer() check.

>   }
>   
> @@ -999,8 +1002,10 @@ static int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg,
>   	if (dev_is_untrusted(dev))
>   		return iommu_dma_map_sg_swiotlb(dev, sg, nents, dir, attrs);
>   
> -	if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC))
> +	if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC)) {
>   		iommu_dma_sync_sg_for_device(dev, sg, nents, dir);
> +		attrs |= DMA_ATTR_SKIP_CPU_SYNC;
> +	}

Why? attrs is never referenced again after this.

>   	/*
>   	 * Work out how much IOVA space we need, and align the segments to
> @@ -1068,8 +1073,10 @@ static void iommu_dma_unmap_sg(struct device *dev, struct scatterlist *sg,
>   	struct scatterlist *tmp;
>   	int i;
>   
> -	if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC))
> +	if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC)) {
>   		iommu_dma_sync_sg_for_cpu(dev, sg, nents, dir);
> +		attrs |= DMA_ATTR_SKIP_CPU_SYNC;
> +	}

Just move it down so it's out of the SWIOTLB path entirely. Exactly like 
you did in patch #2 for map_sg, conspicuous in the hunk above.

Robin.

>   
>   	if (dev_is_untrusted(dev)) {
>   		iommu_dma_unmap_sg_swiotlb(dev, sg, nents, dir, attrs);
>