linux-kernel - Re: [PATCH] iommu/vt-d: Return the correct dma mask when we are bypassing the IOMMU

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Date:   Wed, 9 Oct 2019 10:45:15 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>, linux-kernel@...r.kernel.org
Cc:     baolu.lu@...ux.intel.com, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH] iommu/vt-d: Return the correct dma mask when we are
 bypassing the IOMMU

Hi,

On 10/8/19 10:33 PM, Arvind Sankar wrote:
> We must return a mask covering the full physical RAM when bypassing the
> IOMMU mapping. Also, in iommu_need_mapping, we need to check using
> dma_direct_get_required_mask to ensure that the device's dma_mask can
> cover physical RAM before deciding to bypass IOMMU mapping.
> 
> Fixes: 249baa547901 ("dma-mapping: provide a better default ->get_required_mask")
> Reported-by: Arvind Sankar <nivedita@...m.mit.edu>
> Tested-by: Arvind Sankar <nivedita@...m.mit.edu>
> Originally-by: Christoph Hellwig <hch@....de>
> Signed-off-by: Christoph Hellwig <hch@....de>
> Fixed-by: Arvind Sankar <nivedita@...m.mit.edu>
> Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
> ---
>   drivers/iommu/intel-iommu.c | 10 +++++++++-
>   1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
> index 3f974919d3bd..79e35b3180ac 100644
> --- a/drivers/iommu/intel-iommu.c
> +++ b/drivers/iommu/intel-iommu.c
> @@ -3471,7 +3471,7 @@ static bool iommu_need_mapping(struct device *dev)
>   		if (dev->coherent_dma_mask && dev->coherent_dma_mask < dma_mask)
>   			dma_mask = dev->coherent_dma_mask;
>   
> -		if (dma_mask >= dma_get_required_mask(dev))
> +		if (dma_mask >= dma_direct_get_required_mask(dev))
>   			return false;
>   
>   		/*
> @@ -3775,6 +3775,13 @@ static int intel_map_sg(struct device *dev, struct scatterlist *sglist, int nele
>   	return nelems;
>   }
>   
> +static u64 intel_get_required_mask(struct device *dev)
> +{
> +	if (!iommu_need_mapping(dev))
> +		return dma_direct_get_required_mask(dev);
> +	return DMA_BIT_MASK(32);

Do you mind explaining why we always return 32 bit here?

Best regards,
Baolu

> +}
> +
>   static const struct dma_map_ops intel_dma_ops = {
>   	.alloc = intel_alloc_coherent,
>   	.free = intel_free_coherent,
> @@ -3787,6 +3794,7 @@ static const struct dma_map_ops intel_dma_ops = {
>   	.dma_supported = dma_direct_supported,
>   	.mmap = dma_common_mmap,
>   	.get_sgtable = dma_common_get_sgtable,
> +	.get_required_mask = intel_get_required_mask,
>   };
>   
>   static void
>