Re: [v3 PATCH] iommu/arm-smmu-v3: Fix L1 stream table index calculation for 32-bit sid size

[Daniel Mentz]

On Fri, Oct 4, 2024 at 11:04 AM Yang Shi <yang@xxxxxxxxxxxxxxxxxxxxxx> wrote:
>  static int arm_smmu_init_strtab_linear(struct arm_smmu_device *smmu)
>  {
> -       u32 size;
> +       u64 size;
>         struct arm_smmu_strtab_cfg *cfg = &smmu->strtab_cfg;
> +       u64 num_sids = arm_smmu_strtab_num_sids(smmu);
> +
> +       size = num_sids * sizeof(struct arm_smmu_ste);
> +       /* The max size for dmam_alloc_coherent() is 32-bit */

I'd remove this comment. I assume the intent here was to say that the
maximum size is 4GB (not 32 bit). I also can't find any reference to
this limitation. Where does dmam_alloc_coherent() limit the size of an
allocation to 4GB? Also, this comment might not be applicable to 64
bit platforms.

> +       if (size > SIZE_MAX)
> +               return -EINVAL;

I'm assuming this is for platforms where the range of a u64 is larger
than that of a size_t type? If we're printing an error message if an
allocation fails (i.e. "failed to allocate linear stream table (%llu
bytes)\n"), then we might also want to print an error message here.

> -       cfg->linear.num_ents = 1 << smmu->sid_bits;
> +       cfg->linear.num_ents = num_sids;

If you're worried about 32 bit platforms, then I'm wondering if this
also needs some attention. cfg->linear.num_ents is defined as an
unsigned int and num_sids could potentially be outside the range of an
unsigned int on 32 bit platforms.

> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h
> index 1e9952ca989f..c8ceddc5e8ef 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h
> @@ -853,6 +853,11 @@ struct arm_smmu_master_domain {
>         ioasid_t ssid;
>  };
>
> +static inline u64 arm_smmu_strtab_num_sids(struct arm_smmu_device *smmu)
> +{
> +       return (1ULL << smmu->sid_bits);
> +}
> +

I'm wondering if it makes sense to move this up and put it right
before arm_smmu_strtab_l1_idx(). That way, all the arm_smmu_strtab_*
functions are in one place.

On a related note, in arm_smmu_init_strtab_2lvl() we're capping the
number of l1 entries at STRTAB_MAX_L1_ENTRIES for 2 level stream
tables. I'm thinking it would make sense to limit the size of linear
stream tables for the same reasons.