Re: [QUESTION] Is the ARM SMMU v3 implementation designed to always ignore SSID when SSID_VALID == 0?

[Joonwon Kang <joonwonkang@google.com>]

> On 28/04/2026 12:14 pm, Joonwon Kang wrote:
> > Thanks for your prompt and insightful answer!
> > 
> >> Hi,
> >>
> >> On Tue, Apr 28, 2026 at 07:38:59AM +0000, Joonwon Kang wrote:
> >>> Hi team,
> >>>
> >>> According to the ARM SMMU v3 spec, I believe that SSID should always be
> >>> ignored when SSID_VALID == 0 and the current ARM SMMU v3 module
> >>> implementation in the kernel seems to comply with this without exception.
> >>> For example, when handling an event from SMMU, the implementation checks
> >>> SSID_VALID(SSV) first and ignores SSID accordingly. If there is any
> >>> exception to this rule, I believe it is a bug.
> >>
> >> Indeed
> > 
> > Acknowledged.
> > 
> >>
> >>> Is it true for all the current and future cases? In other words, is it
> >>> **mandatory** that the ARM SMMU v3 implementation ignores SSID when
> >>> SSID_VALID == 0? or there might be some cases where the implementation
> >>> needs to refer to SSID even when SSID_VALID == 0?
> >>>
> >>> Asking this question since our HW may not be able to clear SSID when
> >>> SSID_VALID == 0 and so there might be some garbage value in SSID at some
> >>> point of time(the HW will have a correct SSID when SSID_VALID == 1,
> >>> though). If the ARM SMMU v3 implementation is to refer to that garbage
> >>> value for any reason, the result would be devastating.
> >>
> >> At least according to the architecture, SubstreamID is ignored when SSV=0.
> >> The SMMU is allowed to propagate the garbage:
> >>
> >>    7.3 Event record
> >>
> >>    * SSV: The SubstreamID validity flag
> >>      - 0: No SubstreamID was provided with the transaction and the SubstreamID field is UNKNOWN.
> >>
> >> But the driver will ignore it.
> >>
> >> Same for PRI queue but in that case the page request wouldn't have a PASID
> >> TLP prefix.
> > 
> > Although the PRI request without PASID may cause unpleasant ATC flush with
> > SSV clear in this case, it does not lead to the implementation referring
> > to the garbage SSID. Is this understanding correct? And while this case
> > seems to be handled solely by the ARM SMMU v3 implementation, do you see
> > if there is additional care required on our device driver for this?
> 
> A transaction with SSV==0 does not have a SubstreamID, therefore by 
> definition there is nothing that an SMMU could validly attempt to do 
> with a SubstreamID that does not exist. Sure, implementations can have 
> bugs, but I'd expect any such bug in this regard should be sufficiently 
> obvious that it most likely wouldn't get past architectural validation 
> in the first place.
> 

It makes a lot of sense.

> If you want to know the exact behaviour of Arm's implementations then 
> you're best off asking Arm support, but since this piqued my curiosity 
> too, I can save you the trouble - I checked with my contacts on the 
> design team, and indeed our SMMUs should ignore the SSID value entirely 
> when SSV==0 and just treat it as 0 (e.g. in event records).

Thank you very much for saving me the effort and confirming this. I also
acknowledge that it will be best to ask the Arm support with regards to
the exact behavior of Arm's implementation. 

Thanks,
Joonwon Kang