Re: [PATCH] iommu/vt-d: Fix scatterlist offset handling

[Jacob Pan]

Hi Alex and all,

Just wondering if you could merge Robin's patch for the next rc. From
all our testing, this seems to be a solid fix and should be included in
the stable releases as well.

Thanks,

Jacob

On Mon, 6 Nov 2017 10:47:09 -0800
Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx> wrote:

> On Fri, 6 Oct 2017 16:43:09 +0200
> Joerg Roedel <joro@xxxxxxxxxx> wrote:
> 
> > On Tue, Oct 03, 2017 at 07:05:17PM +0100, Robin Murphy wrote:  
> > > Now, there are indeed plenty of drivers and subsystems which do
> > > work on lists of explicitly single pages - anything doing some
> > > variant of "addr = kmap_atomic(sg_page(sg)) + sg->offset;" is easy
> > > to spot - but I don't think DMA API implementations are in a
> > > position to make any kind of assumption; nearly all of them just
> > > shut up and handle sg->length bytes from sg_phys(sg) without
> > > questioning the caller, and I reckon that's exactly what they
> > > should be doing.    
> > 
> > I agree with that, it is not explicitly forbidden to have an
> > sg->offset > PAGE_SIZE and most IOMMU drivers handle this case.
> > 
> > So this is a problem I'd like to see resolved in the VT-d driver
> > too. If nobody comes up with a correct fix soon I'll apply this one
> > and rip out the large-page support from __domain_mapping() to make
> > it work.
> >   
> Hi All,
> 
> Just to give an update on the offline debugging of this issue. With
> Robin's patch applied, I was able to reproduce the failure with
> similar configuration that Jain helped to set up.
> 
> I added trace prints just to see the map/unmap activities leading to
> the DMAR fault. When fault occurs, the trace shows there is an unmap
> to the offending iova pfn. So I think this is a separate problem than
> Robin's patch is fixing. I think we should move forward to merge this
> patch upstream and stable. The remaining problem is likely a race
> condition between unmap and DMA activities.
> 
> Here a brief extracted log, ee3d7 is the iova pfn in question.
> #1. map sg pfn ee3d7
>           <idle>-0     [076] 74124.154254: bprint:
> __domain_mapping: vpfn:ee3d7, pgoff=2126, np:1, da:ee3d784e,
> len:1464 ,
> ppfn:1849c9c                                                                                                                        
> 
> #2. unmap ee3d7000
>          <idle>-0     [054] 74124.154301: bprint:
> intel_unmap: Device 0000:18:00.4 unmapping: pfn ee3d7-ee3d7
> <idle>-0     [076] 74124.154301: bprint:
> __domain_mapping: lvlpg:1, nrpg 0, vpfn:ec2ff, ppfn:183221a, sg_res:0
> <idle>-0     [059] 74124.154302: bprint:
> __domain_mapping: lvlpg:1, nrpg 0, vpfn:ee719, ppfn:c3e4dd, sg_res:0
> <idle>-0     [076] 74124.154302: bprint:
> __domain_mapping: vpfn:f183b, pgoff=78, np:1, da:f183b04e, len:1464, 
> 
> #3. DMA to unmapped address ee3d7000, DMAR fault raised.
>   +2.952861] dmar_fault: 6 callbacks
> suppressed +0.000002] DMAR: DRHD: handling fault status reg
> 2 +0.005588] turning tracing
> off +0.003592] DMAR: [DMA Write] Request device [18:00.4] fault addr
> ee3d7000 [fault reason 05] PTE Write access is not set 
>                                                                                                                                                                                                                                                                
>          <idle>-0     [000] 74124.156906: bputs:
>          0xffffffffb259916bs: turning tracing off     
> 
> 
> Thanks,
> 
> Jacob
> 
> > Speaking of __domain_mapping(), this function is a big
> > unmaintainable mess which should be split and rewritten. A clean
> > and maintainable rewrite can alse re-add the large-page support.
> > 
> > 
> > Regards,
> > 
> > 	Joerg
> > 
> > _______________________________________________
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> 
> [Jacob Pan]

[Jacob Pan]