Re: [PATCH 1/1] iommu/vt-d: use DMA domain for real DMA devices and subdevices

[Lu Baolu <baolu.lu@linux.intel.com>]

Hi,

On 2020/4/10 3:17, Jon Derrick wrote:
> The PCI devices handled by intel-iommu may have a DMA requester on another bus,
> such as VMD subdevices needing to use the VMD endpoint.
> 
> The real DMA device is now used for the DMA mapping, but one case was missed
> earlier: if the VMD device (and hence subdevices too) are under
> IOMMU_DOMAIN_IDENTITY, mappings do not work.
> 
> Codepaths like intel_map_page() handle the IOMMU_DOMAIN_DMA case by creating an
> iommu DMA mapping, and fall back on dma_direct_map_page() for the
> IOMMU_DOMAIN_IDENTITY case. However, handling of the IDENTITY case is broken
> when intel_page_page() handles a subdevice.
> 
> We observe that at iommu attach time, dmar_insert_one_dev_info() for the
> subdevices will never set dev->archdata.iommu. This is because that function

Do you mind telling why not setting this?

> uses find_domain() to check if there is already an IOMMU for the device, and
> find_domain() then defers to the real DMA device which does have one. Thus
> dmar_insert_one_dev_info() returns without assigning dev->archdata.iommu.
> 
> Then, later:
> 
> 1. intel_map_page() checks if an IOMMU mapping is needed by calling
>     iommu_need_mapping() on the subdevice. identity_mapping() returns
>     false because dev->archdata.iommu is NULL, so this function
>     returns false indicating that mapping is needed.
> 2. __intel_map_single() is called to create the mapping.
> 3. __intel_map_single() calls find_domain(). This function now returns
>     the IDENTITY domain corresponding to the real DMA device.
> 4. __intel_map_single() calls domain_get_iommu() on this "real" domain.
>     A failure is hit and the entire operation is aborted, because this
>     codepath is not intended to handle IDENTITY mappings:
>         if (WARN_ON(domain->domain.type != IOMMU_DOMAIN_DMA))
>                     return NULL;

This is caused by the fragile private domain implementation. We are in
process of removing it by enhancing the iommu subsystem with per-group
default domain.

https://www.spinics.net/lists/iommu/msg42976.html

So ultimately VMD subdevices should have their own per-device iommu data
and support per-device dma ops.

Best regards,
baolu