On Tue, Jan 27, 2026 at 01:50:54PM +0000, Will Deacon wrote:

Argh, I had conflated a transaction using SSID 0 vs a transaction
without a substream at all. So I think this makes sense now...

Yeah, it is bit subtle, but as a SW choice the iommu subsystem
reserves PASID 0/SSID 0 as the "untagged" translation.

Several HW's force this in their implementation (ie AMD)

ARM however includes a "Substream Valid" in the input bus. Linux
doesn't use the combination "Substream Valid, SSID=0", that should
never occur.

If it wrongly does happen then IDENTITY will generate a fault, either
C_BAD_CD (due to it being non-valid) or C_BAD_SUBSTREAMID (due to
S1CDMax disabling substreams).

While PAGING will either fault with C_BAD_SUBSTREAMID (S2 paging
domain) or success when S1DSS=b10.

Jason