linux-kernel - RE: [PATCH v3 5/5] dma-iommu: account for min_align

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Date:   Wed, 11 Aug 2021 09:26:02 +0000
From:   "Mi, Dapeng1" <dapeng1.mi@...el.com>
To:     David Stevens <stevensd@...omium.org>,
        Robin Murphy <robin.murphy@....com>,
        Will Deacon <will@...nel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Tom Murphy <murphyt7@....ie>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>
Subject: RE: [PATCH v3 5/5] dma-iommu: account for min_align_mask

> -----Original Message-----
> From: iommu <iommu-bounces@...ts.linux-foundation.org> On Behalf Of
> David Stevens
> Sent: Wednesday, August 11, 2021 10:43 AM
> To: Robin Murphy <robin.murphy@....com>; Will Deacon <will@...nel.org>
> Cc: linux-kernel@...r.kernel.org; Tom Murphy <murphyt7@....ie>;
> iommu@...ts.linux-foundation.org; David Stevens <stevensd@...omium.org>
> Subject: [PATCH v3 5/5] dma-iommu: account for min_align_mask
> 
> From: David Stevens <stevensd@...omium.org>
> 
> For devices which set min_align_mask, swiotlb preserves the offset of the
> original physical address within that mask. Since __iommu_dma_map
> accounts for non-aligned addresses, passing a non-aligned swiotlb address
> with the swiotlb aligned size results in the offset being accounted for twice in
> the size passed to iommu_map_atomic. The extra page exposed to DMA is
> also not cleaned up by __iommu_dma_unmap, since tht at function unmaps
> with the correct size. This causes mapping failures if the iova gets reused,
> due to collisions in the iommu page tables.
> 
> To fix this, pass the original size to __iommu_dma_map, since that function
> already handles alignment.
> 
> Additionally, when swiotlb returns non-aligned addresses, there is padding at
> the start of the bounce buffer that needs to be cleared.
> 
> Fixes: 1f221a0d0dbf ("swiotlb: respect min_align_mask")
> Signed-off-by: David Stevens <stevensd@...omium.org>
> ---
>  drivers/iommu/dma-iommu.c | 23 ++++++++++++-----------
>  1 file changed, 12 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index 89b689bf801f..ffa7e8ef5db4 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -549,9 +549,8 @@ static dma_addr_t
> __iommu_dma_map_swiotlb(struct device *dev, phys_addr_t phys,
>  	struct iommu_domain *domain = iommu_get_dma_domain(dev);
>  	struct iommu_dma_cookie *cookie = domain->iova_cookie;
>  	struct iova_domain *iovad = &cookie->iovad;
> -	size_t aligned_size = org_size;
> -	void *padding_start;
> -	size_t padding_size;
> +	void *tlb_start;
> +	size_t aligned_size, iova_off, mapping_end_off;
>  	dma_addr_t iova;
> 
>  	/*
> @@ -566,24 +565,26 @@ static dma_addr_t
> __iommu_dma_map_swiotlb(struct device *dev, phys_addr_t phys,
>  		if (phys == DMA_MAPPING_ERROR)
>  			return DMA_MAPPING_ERROR;
> 
> -		/* Cleanup the padding area. */
> -		padding_start = phys_to_virt(phys);
> -		padding_size = aligned_size;
> +		iova_off = iova_offset(iovad, phys);
> +		tlb_start = phys_to_virt(phys - iova_off);
> 
> +		/* Cleanup the padding area. */
>  		if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC) &&
>  		    (dir == DMA_TO_DEVICE ||
>  		     dir == DMA_BIDIRECTIONAL)) {
> -			padding_start += org_size;
> -			padding_size -= org_size;
> +			mapping_end_off = iova_off + org_size;
> +			memset(tlb_start, 0, iova_off);
> +			memset(tlb_start + mapping_end_off, 0,
> +			       aligned_size - mapping_end_off);
> +		} else {
> +			memset(tlb_start, 0, aligned_size);
>  		}

Nice fix. It's better move the "cleanup ..." comment into if case which looks more accurate.