RE: [PATCH kernel] powerpc/iommu: Add iommu_ops to report capabilities and allow blocking domains

From: Alexey Kardashevskiy <redacted>
Sent: Friday, July 8, 2022 2:35 PM
On 7/8/22 15:00, Alexey Kardashevskiy wrote:

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On 7/8/22 01:10, Jason Gunthorpe wrote:

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On Thu, Jul 07, 2022 at 11:55:52PM +1000, Alexey Kardashevskiy wrote:

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Historically PPC64 managed to avoid using iommu_ops. The VFIO driver
uses a SPAPR TCE sub-driver and all iommu_ops uses were kept in
the Type1 VFIO driver. Recent development though has added a

coherency

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capability check to the generic part of VFIO and essentially disabled
VFIO on PPC64; the similar story about

iommu_group_dma_owner_claimed().

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This adds an iommu_ops stub which reports support for cache
coherency. Because bus_set_iommu() triggers IOMMU probing of PCI
devices,
this provides minimum code for the probing to not crash.

stale comment since this patch doesn't use bus_set_iommu() now.

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+
+static int spapr_tce_iommu_attach_dev(struct iommu_domain *dom,
+                      struct device *dev)
+{
+    return 0;
+}

It is important when this returns that the iommu translation is all
emptied. There should be no left over translations from the DMA API at
this point. I have no idea how power works in this regard, but it
should be explained why this is safe in a comment at a minimum.

 > It will turn into a security problem to allow kernel mappings to leak
 > past this point.
 >

I've added for v2 checking for no valid mappings for a device (or, more
precisely, in the associated iommu_group), this domain does not need
checking, right?

Uff, not that simple. Looks like once a device is in a group, its
dma_ops is set to iommu_dma_ops and IOMMU code owns DMA. I guess
then
there is a way to set those to NULL or do something similar to let
dma_map_direct() from kernel/dma/mapping.c return "true", is not there?

dev->dma_ops is NULL as long as you don't handle DMA domain type
here and don't call iommu_setup_dma_ops().

Given this only supports blocking domain then above should be irrelevant.

For now I'll add a comment in spapr_tce_iommu_attach_dev() that it is
fine to do nothing as tce_iommu_take_ownership() and
tce_iommu_take_ownership_ddw() take care of not having active DMA
mappings. Thanks,

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In general, is "domain" something from hardware or it is a software
concept? Thanks,

'domain' is a software concept to represent the hardware I/O page
table. A blocking domain means all DMAs from a device attached to
this domain are blocked/rejected (equivalent to an empty I/O page
table), usually enforced in the .attach_dev() callback. 

Yes, a comment for why having a NULL .attach_dev() is OK is welcomed.

Thanks
Kevin