Re: question about iommu_need_mapping

[Jerry Snitselaar]

On Thu Feb 20 20, Jerry Snitselaar wrote:
> On Thu Feb 20 20, Lu Baolu wrote:
> > Hi Jerry,
> >
> > On 2020/2/20 7:55, Jerry Snitselaar wrote:
> > > Is it possible for a device to end up with dev->archdata.iommu == NULL
> > > on iommu_need_mapping in the following instance:
> > >
> > > 1. iommu_group has dma domain for default
> > > 2. device gets private identity domain in intel_iommu_add_device
> > > 3. iommu_need_mapping gets called with that device.
> > > 4. dmar_remove_one_dev_info sets dev->archdata.iommu = NULL via 
> > > unlink_domain_info.
> > > 5. request_default_domain_for_dev exits after checking that 
> > > group->default_domain
> > > ÂÂ exists, and group->default_domain->type is dma.
> > > 6. iommu_request_dma_domain_for_dev returns 0 from 
> > > request_default_domain_for_dev
> > > ÂÂ and a private dma domain isn't created for the device.
> >
> > Yes. It's possible.
> >
> > > The case I was seeing went away with commit 9235cb13d7d1 ("iommu/vt-d:
> > > Allow devices with RMRRs to use identity domain"), because it changed
> > > which domain the group and devices were using, but it seems like it is
> > > still a possibility with the code. Baolu, you mentioned possibly
> > > removing the domain switch. Commit 98b2fffb5e27 ("iommu/vt-d: Handle
> > > 32bit device with identity default domain") makes it sound like the
> > > domain switch is required.
> >
> > It's more "nice to have" than "required" if the iommu driver doesn't
> > disable swiotlb explicitly. The device access of system memory higher
> > than the device's addressing capability could go through the bounced
> > buffer implemented in swiotlb.
> >
> > Best regards,
> > baolu
>
> Hi Baolu,
>
> Would this mean switching to bounce_dma_ops instead?

Never mind. I see that it would go into the dma_direct code.

> Regards,
> Jerry
>
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