Re: [PATCH RFC v2 02/11] iommu: Add iommu_group_singleton_lockdown()

[Jason Gunthorpe via iommu <iommu@lists.linux-foundation.org>]

On Tue, Mar 29, 2022 at 08:42:13AM +0000, Tian, Kevin wrote:

> btw I'm not sure whether this is what SVA requires. IIRC the problem with
> SVA is because PASID TLP prefix is not counted in PCI packet routing thus
> a DMA target address with PASID might be treated as P2P if the address
> falls into the MMIO BAR of other devices in the group. This is why the
> original code needs to strictly apply SVA in a group containing a single
> device, instead of a group attached by a single driver, unless we want to
> reserve those MMIO ranges in CPU VA space.

I think it is not such a good idea to mix up group with this test

Here you want to say that all TLPs from the RID route to the host
bridge - ie ACS is on/etc. This is subtly different from a group with
a single device. Specifically it is an immutable property of the
fabric and doesn't change after hot plug events.

ie if we have a singleton group that doesn't have ACS and someone
hotplugs in another device on a bridge, then our SVA is completely
broken and we get data corruption.

Testing the group size is inherently the wrong test to make.

Jason
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