Re: [PATCH v2] iommu/arm-smmu-v3: Align memory attributes for SMMU-originated accesses

[Jason Gunthorpe]

On Sun, Oct 04, 2026 at 07:50:27PM +0000, Daniel Mentz wrote:
> The SMMU specification defines several types of SMMU-originated memory
> accesses, including Stage 1 and Stage 2 translation table walks, stream
> table accesses (L1STD and STE fetches), CD table accesses (L1CD and CD
> fetches), and queue accesses (CMDQ fetch, EVENTQ write, PRIQ write).
> 
> While the memory attributes used for Stage 1 and Stage 2 translation
> table walks are configured in io-pgtable-arm based on whether the SMMU
> is coherent (ARM_SMMU_FEAT_COHERENCY), the attributes for stream tables,
> CD tables, and queues are currently hardcoded to Inner Shareable,
> Write-Back.
> 
> On non-coherent systems, however, memory for stream tables, CD tables,
> and queues is allocated via dma_alloc_coherent() /
> dmam_alloc_coherent(), which provides CPU mappings with Normal
> Non-Cacheable attributes. Having a non-coherent SMMU access these
> buffers with Inner Shareable, Write-Back attributes results in
> mismatched memory attributes between the CPU and the SMMU.
> 
> Configure the memory attributes for tables and queues in
> arm_smmu_device_reset() and arm_smmu_make_cdtable_ste() based on
> ARM_SMMU_FEAT_COHERENCY, matching the attributes used for translation
> table walks:
> 
> - In SMMU_CR1, use Outer Shareable, Non-Cacheable for non-coherent
>   SMMUs, while retaining Inner Shareable, Write-Back for coherent SMMUs.
>   This applies to stream table accesses as well as queue accesses (CMDQ
>   fetch, EVENTQ write, PRIQ write).
> - In STE.{S1CIR, S1COR, S1CSH}, use Outer Shareable, Non-Cacheable for
>   non-coherent SMMUs, while retaining Inner Shareable, Write-Back
>   Read-Allocate for coherent SMMUs. This applies to CD table accesses.
> 
> Assisted-by: LLM
> Reviewed-by: Nicolin Chen <nicolinc@xxxxxxxxxx>
> Signed-off-by: Daniel Mentz <danielmentz@xxxxxxxxxx>
> ---
> Changes in v2:
> - Rename local variables 'cache' and 'sh' to 'cr1_cache' and 'cr1_sh' in
>   arm_smmu_device_reset() (Nicolin, Will)
> - Collect Reviewed-by from Nicolin
> - Link to v1: https://lore.kernel.org/linux-iommu/20260929032229.3532247-1-danielmentz@xxxxxxxxxx/
> 
>  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 37 +++++++++++++++------
>  1 file changed, 27 insertions(+), 10 deletions(-)

I think this looks OK

Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>

Did you check the spec that this is every access? I remember there is
a table outlining every access and where the memory attribute lives?

Jason