Re: question about iommu_need_mapping

[Jerry Snitselaar]

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?

Regards,
Jerry

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