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

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Date:   Tue, 26 Sep 2017 19:15:47 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     "Raj, Ashok" <ashok.raj@...el.com>
Cc:     Harsh Jain <Harsh@...lsio.com>, Casey Leedom <leedom@...lsio.com>,
        Herbert Xuy <herbert@...dor.apana.org.au>,
        David Woodhouse <David.Woodhouse@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>
Subject: Re: DMA error when sg->offset value is greater than PAGE_SIZE in
 Intel IOMMU

On 26/09/17 15:34, Raj, Ashok wrote:
> On Tue, Sep 26, 2017 at 03:22:47PM +0100, Robin Murphy wrote:
>> diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
>> index 6784a05dd6b2..d7f7def81613 100644
>> --- a/drivers/iommu/intel-iommu.c
>> +++ b/drivers/iommu/intel-iommu.c
>> @@ -2254,10 +2254,12 @@ static int __domain_mapping(struct dmar_domain *domain, unsigned long iov_pfn,
>>  		uint64_t tmp;
>>  
>>  		if (!sg_res) {
>> +			size_t off = sg->offset & ~PAGE_MASK;
> 
> Should this be VTD_PAGE_MASK?

PAGE_MASK (and the corresponding pteval arithmetic) was intentional
here; given the way aligned_nrpages() works, the IOVA space allocated in
intel_map_sg() (and thus iov_pfn) is already rounded to full MM pages,
and it seemed like the original intent was to map the whole lot - this
change is just to make that happen correctly.

Whether it's actually reasonable to decouple the IOMMU and CPU page
sizes entirely (as we do in dma-iommu, for example), and not do the
MM-page-alignment thing at all, is another matter that I'm happy to
leave in your hands :)

Robin.

>> +
>>  			sg_res = aligned_nrpages(sg->offset, sg->length);
>> -			sg->dma_address = ((dma_addr_t)iov_pfn << VTD_PAGE_SHIFT) + sg->offset;
>> +			sg->dma_address = ((dma_addr_t)iov_pfn << VTD_PAGE_SHIFT) + off;
>>  			sg->dma_length = sg->length;
>> -			pteval = page_to_phys(sg_page(sg)) | prot;
>> +			pteval = (page_to_phys(sg_page(sg)) + sg->offset - off) | prot;
> 
> Something seems wrong here.. sg->offset can be > VTD_PAGE_SIZE, think 
> we should add sg->offset and then find the pteval?
> 
> attached below another cut at fixing the same problem.. if there is something
> obvious i missed, let me know.
> 
> again.. untested :-)
> 
> Cheers,
> Ashok
>