linux-kernel - Re: DMA error when sg->offset value is greater than PAGE

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Date:   Wed, 20 Sep 2017 16:50:24 +0530
From:   Harsh Jain <Harsh@...lsio.com>
To:     Robin Murphy <robin.murphy@....com>,
        Herbert Xu <herbert@...dor.apana.org.au>
Cc:     leedom@...lsio.com, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, linux-crypto@...r.kernel.org,
        dwmw2@...radead.org
Subject: Re: DMA error when sg->offset value is greater than PAGE_SIZE in
 Intel IOMMU



On 20-09-2017 15:42, Robin Murphy wrote:
> On 20/09/17 09:01, Herbert Xu wrote:
>> Harsh Jain <Harsh@...lsio.com> wrote:
>>> While debugging DMA mapping error in chelsio crypto driver we observed that when scatter/gather list received by driver has some entry with page->offset > 4096 (PAGE_SIZE). It starts giving DMA error.  Without IOMMU it works fine.
>> This is not a bug.  The network stack can and will feed us such
>> SG lists.
>>
>>> 2) It cannot be driver's responsibilty to update received sg entries to adjust offset and page 
>>> because we are not the only one who directly uses received sg list.
>> No the driver must deal with this.  Having said that, if we can
>> improve our driver helper interface to make this easier then we
>> should do that too.  What we certainly shouldn't do is to take a
>> whack-a-mole approach like this patch does.
> AFAICS this is entirely on intel-iommu - from a brief look it appears
> that all the IOVA calculations would handle the offset correctly, but
> then __domain_mapping() blindly uses sg_page() for the physical address,
> so if offset is larger than a page it would end up with the DMA mapping
> covering the wrong part of the buffer.
>
> Does the diff below help?
>
> Robin.
>
> ----->8-----
> diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
> index b3914fce8254..2ed43d928135 100644
> --- a/drivers/iommu/intel-iommu.c
> +++ b/drivers/iommu/intel-iommu.c
> @@ -2253,7 +2253,7 @@ static int __domain_mapping(struct dmar_domain *domain, unsigned long iov_pfn,
>  			sg_res = aligned_nrpages(sg->offset, sg->length);
>  			sg->dma_address = ((dma_addr_t)iov_pfn << VTD_PAGE_SHIFT) + sg->offset;
>  			sg->dma_length = sg->length;
> -			pteval = page_to_phys(sg_page(sg)) | prot;
> +			pteval = (sg_phys(sg) & PAGE_MASK) | prot;
>  			phys_pfn = pteval >> VTD_PAGE_SHIFT;
>  		}
Robin,
Still having following error with above change.

[  429.645492] DMAR: DRHD: handling fault status reg 2
[  429.650847] DMAR: [DMA Write] Request device [02:00.4] fault addr f2682000 [t


>