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
