linux-kernel - Re: [PATCH RFCv1 3/3] iommu/arm-smmu-v3: Allow ATS to be always on

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Message-ID: <f08595ff-f36e-45cc-8ea2-dc678f59c7d9@arm.com>
Date: Tue, 27 Jan 2026 13:26:02 +0000
From: Robin Murphy <robin.murphy@....com>
To: Will Deacon <will@...nel.org>, Jason Gunthorpe <jgg@...dia.com>
Cc: Nicolin Chen <nicolinc@...dia.com>, bhelgaas@...gle.com, joro@...tes.org,
 praan@...gle.com, baolu.lu@...ux.intel.com, kevin.tian@...el.com,
 miko.lenczewski@....com, linux-arm-kernel@...ts.infradead.org,
 iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
 linux-pci@...r.kernel.org
Subject: Re: [PATCH RFCv1 3/3] iommu/arm-smmu-v3: Allow ATS to be always on

On 2026-01-27 1:10 pm, Will Deacon wrote:
> On Mon, Jan 26, 2026 at 03:09:35PM -0400, Jason Gunthorpe wrote:
>> On Mon, Jan 26, 2026 at 06:49:07PM +0000, Robin Murphy wrote:
>>> (assuming SSIDSIZE > 0 and it does anything at all - note that strictly we
>>> cannot assume this bypass trick is *always* possible, since an SMMU is
>>> permitted to support ATS without supporting SubStreams).
>>
>> Yes, I think Nicolin has captured those conditions in computing
>> it... We don't have a logic to disable bypass in that case though.
>>
>>>> So, I think a CD table pointer to a fully invalid L1 table of at least
>>>> size 1 should be OK?
>>>>
>>>> Or stated another way, why would ie be OK to have a 1 level table with
>>>> an non-valid CD table entry for SSID0 but not OK to have a 2 level
>>>> table that returns non-valid at the first walk?
>>>
>>> S1ContextPtr itself is reachable since S1 is enabled, so it cannot point to
>>> nonsense. But the S1DSS==Bypass behaviour does state:
>>
>>> "Note: Such a transaction does not fetch a CD, and therefore does not report
>>> F_CD_FETCH, C_BAD_CD or a stage 2 Translation-related fault with CLASS ==
>>> CD."
>>
>> Yes
>>
>> However, taken together:
>>   * S1CDMax is set to substream 0 only
>>   * S1DSS is set such that "does not fetch a CD" for SSID = 0
>>   * SSID >0 doesn't fetch CDs because of S1CDMax
>>
>> Then it seems to be saying that it will never use S1ContextPtr? ie it
>> is IGNORED?
> 
> Right, I think the critical question is whether that setting of S1DSS
> (0b01) means that STE.S1ContextPtr is considered "invalid". The spec
> doesn't call this out explicitly but the "translation procedure charts"
> seem to indicate that it doesn't use the CD for anything...
> 
> It would be good to get some clarification from Arm about this
> particular case.

No, STE.S1ContextPtr itself is "valid" since S1 is enabled. No CD fetch 
will occur for no-SubStreamID transactions that are bypassed by S1DSS, 
but the SMMU is permitted to attempt to speculatively fetch CDs for the 
enabled SubStreamID(s). Those fetches do not have to reach a valid CD if 
the SubStream is not actually in use, much like we don't have to fully 
populate a 2-level Stream table for StreamID ranges we don't care about 
either.

Don't confuse S1DSS==1 (bypass) with the S1DSS==2 behaviour we use in 
other cases - the latter is "Use CD 0 for no-SubstreamID traffic" which 
makes SubStreamID 0 invalid to use. However in the bypass case (and also 
S1DSS==0 where no-SubstreamID traffic is blocked entirely), SubStreamID 
0 remains perfectly valid and usable (we just still won't ever use it in 
Linux due to the middle case).

Thanks,
Robin.