Re: [PATCH] xen/arm: smmuv3: Add cache maintenance for non-coherent SMMU queues

[Julien Grall <julien@xxxxxxx>]

Hi,

On 26/08/2025 10:47, Dmytro Firsov wrote:
> On 22.08.25 11:12, Orzel, Michal wrote:
> On 06/08/2025 16:58, Dmytro Firsov wrote:
>
>
> According to the Arm SMMUv3 spec (ARM IHI 0070), a system may have
> SMMU(s) that is/are non-coherent to the PE (processing element). In such
> cases, memory accesses from the PE should be either non-cached or be
> augmented with manual cache maintenance. SMMU cache coherency is reported
> by bit 4 (COHACC) of the SMMU_IDR0 register and is already present in the
> Xen driver. However, the current implementation is not aware of cache
> maintenance for memory that is shared between the PE and non-coherent
> SMMUs. It contains dmam_alloc_coherent() function, that is added during
> Linux driver porting. But it is actually a wrapper for _xzalloc(), that
> returns normal writeback memory (which is OK for coherent SMMUs).
>
> During Xen bring-up on a system with non-coherent SMMUs, the driver did
> not work properly - the SMMU was not functional and halted initialization
> at the very beginning due to a timeout while waiting for CMD_SYNC
> completion:
>
>    (XEN) SMMUv3: /soc/iommu@fa000000: CMD_SYNC timeout
>    (XEN) SMMUv3: /soc/iommu@fa000000: CMD_SYNC timeout
>
> To properly handle such scenarios, add the non_coherent flag to the
> arm_smmu_queue struct. It is initialized using features reported by the
> SMMU HW and will be used for triggering cache clean/invalidate operations.
> This flag is not queue-specific (it is applicable to the whole SMMU), but
> adding it to arm_smmu_queue allows us to not change function signatures
> and simplify the patch (smmu->features, which contains the required flag,
> are not available in code parts that require cache maintenance).
>
>
> There are already a few places advertising the SMMU coherency:
> 1) smmu->features
> 2) d->iommu->features
> 3) platform_features
>
> All of them are better places than queue struct (that as you pointed out is not
> specific to coherency). I'd suggest maybe to use 3) and removing ro_after_init
> if you don't have access to 1) and 2). All in all, providing yet another place
> for coherency flag seems a bit too much.
> >
> First of all, thank you very much for review! I will consider using
> "platform_features" in next patch version, it looks more appropriate and
> should be available within the whole driver. Also, I believe "ro_after_init"
> is also OK, since I have no need to change it (only check if cache
> maintenance should be performed).
I have to disagree with using "platform_features". Flushing the queue is 
a per-SMMU decision. But looking at the code, I think passing the SMMU 
to the caller would look wrong (what if you mistakenly pass the wrong 
SMMU?). So I think a boolean per queue is the right appraoch.
Cheers,

--
Julien Grall