linux-kernel - Re: [PATCH v3 1/5] dma-iommu: fix sync

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Date:   Wed, 11 Aug 2021 19:22:22 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@...radead.org>,
        David Stevens <stevensd@...omium.org>
Cc:     Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        Tom Murphy <murphyt7@....ie>, iommu@...ts.linux-foundation.org
Subject: Re: [PATCH v3 1/5] dma-iommu: fix sync_sg with swiotlb

On 2021-08-11 06:57, Christoph Hellwig wrote:
> On Wed, Aug 11, 2021 at 11:42:43AM +0900, David Stevens wrote:
>> From: David Stevens <stevensd@...omium.org>
>>
>> The is_swiotlb_buffer function takes the physical address of the swiotlb
>> buffer, not the physical address of the original buffer. The sglist
>> contains the physical addresses of the original buffer, so for the
>> sync_sg functions to work properly when a bounce buffer might have been
>> used, we need to use iommu_iova_to_phys to look up the physical address.
>> This is what sync_single does, so call that function on each sglist
>> segment.
>>
>> The previous code mostly worked because swiotlb does the transfer on map
>> and unmap. However, any callers which use DMA_ATTR_SKIP_CPU_SYNC with
>> sglists or which call sync_sg would not have had anything copied to the
>> bounce buffer.
>>
>> Fixes: 82612d66d51d ("iommu: Allow the dma-iommu api to use bounce buffers")
>> Signed-off-by: David Stevens <stevensd@...omium.org>
>> ---
>>   drivers/iommu/dma-iommu.c | 27 +++++++++++++--------------
>>   1 file changed, 13 insertions(+), 14 deletions(-)
>>
>> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
>> index 98ba927aee1a..54e103b989d9 100644
>> --- a/drivers/iommu/dma-iommu.c
>> +++ b/drivers/iommu/dma-iommu.c
>> @@ -813,14 +813,13 @@ static void iommu_dma_sync_sg_for_cpu(struct device *dev,
>>   	if (dev_is_dma_coherent(dev) && !dev_is_untrusted(dev))
>>   		return;
>>   
>> +	if (dev_is_untrusted(dev))
>> +		for_each_sg(sgl, sg, nelems, i)
>> +			iommu_dma_sync_single_for_cpu(dev, sg_dma_address(sg),
>> +						      sg->length, dir);
>> +	else
>> +		for_each_sg(sgl, sg, nelems, i)
>>   			arch_sync_dma_for_cpu(sg_phys(sg), sg->length, dir);
>>   }
> 
> I'd remove the above check and fold the if (!dev_is_dma_coherent(dev))
> into the else line.  Same for iommu_dma_sync_sg_for_device.

+1

With those also cleaned up,

Reviewed-by: Robin Murphy <robin.murphy@....com>