RE: [PATCH V10 03/11] iommu/vt-d: Add a helper function to skip agaw

["Tian, Kevin" <kevin.tian@intel.com>]

> From: Jacob Pan <jacob.jun.pan@linux.intel.com>
> Sent: Saturday, March 21, 2020 7:28 AM
> 
> Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>

could you elaborate in which scenario this helper function is required?
 
> ---
>  drivers/iommu/intel-pasid.c | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/drivers/iommu/intel-pasid.c b/drivers/iommu/intel-pasid.c
> index 22b30f10b396..191508c7c03e 100644
> --- a/drivers/iommu/intel-pasid.c
> +++ b/drivers/iommu/intel-pasid.c
> @@ -500,6 +500,28 @@ int intel_pasid_setup_first_level(struct intel_iommu
> *iommu,
>  }
> 
>  /*
> + * Skip top levels of page tables for iommu which has less agaw
> + * than default. Unnecessary for PT mode.
> + */
> +static inline int iommu_skip_agaw(struct dmar_domain *domain,
> +				  struct intel_iommu *iommu,
> +				  struct dma_pte **pgd)
> +{
> +	int agaw;
> +
> +	for (agaw = domain->agaw; agaw > iommu->agaw; agaw--) {
> +		*pgd = phys_to_virt(dma_pte_addr(*pgd));
> +		if (!dma_pte_present(*pgd)) {
> +			return -EINVAL;
> +		}
> +	}
> +	pr_debug_ratelimited("%s: pgd: %llx, agaw %d d_agaw %d\n",
> __func__, (u64)*pgd,
> +		iommu->agaw, domain->agaw);
> +
> +	return agaw;
> +}
> +
> +/*
>   * Set up the scalable mode pasid entry for second only translation type.
>   */
>  int intel_pasid_setup_second_level(struct intel_iommu *iommu,
> --
> 2.7.4