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

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Message-ID: <0a60797a-fbb9-4a83-adc6-ad62baf5915b@arm.com>
Date: Mon, 26 Jan 2026 18:49:07 +0000
From: Robin Murphy <robin.murphy@....com>
To: Jason Gunthorpe <jgg@...dia.com>, Will Deacon <will@...nel.org>
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 2026-01-26 5:20 pm, Jason Gunthorpe wrote:
> On Mon, Jan 26, 2026 at 12:39:50PM +0000, Will Deacon wrote:
>>> +	ret = arm_smmu_alloc_cd_tables(master);
>>> +	if (ret)
>>> +		return ret;
>>
>> Were do you allocate the second level entry for ssid 0 if we're using
>> 2-level cd tables?
> 
> I don't think we need to. The entire design here has a non-valid CD entry
> for SSID 0.
> 
> The spec is really weird here, on one hand it explicitly says that with
> S1DSS the CD entry is ignored.
> 
> On the other hand, you are also required to have a CD table pointer of
> at least size one for some reason.

Because it is not possible to enable 0 SubStreams, since that wouldn't 
make any sense, hence S1CDMax also acts as the "enable SubStreams" 
control (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).

> 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."

So if we're not intending to actually allow traffic on the SubStream(s), 
then it should be fine to use either a 1-level table of invalid CDs, or 
a 2-level format with an empty L1CD table to gracefully terminate any 
config prefetches.

Thanks,
Robin.