Re: [PATCH v3 3/3] arm-smmu: select suitable MSI IOVA

[Jason Gunthorpe <jgg@ziepe.ca>]

On Wed, Aug 06, 2025 at 02:55:39PM -0700, Shyam Saini wrote:
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> @@ -3642,17 +3642,30 @@ static int arm_smmu_of_xlate(struct device *dev,
>  static void arm_smmu_get_resv_regions(struct device *dev,
>  				      struct list_head *head)
>  {
> -	struct iommu_resv_region *region;
>  	int prot = IOMMU_WRITE | IOMMU_NOEXEC | IOMMU_MMIO;
>  
> -	region = iommu_alloc_resv_region(MSI_IOVA_BASE, MSI_IOVA_LENGTH,
> -					 prot, IOMMU_RESV_SW_MSI, GFP_KERNEL);
> -	if (!region)
> -		return;
> -
> -	list_add_tail(&region->list, head);
> +	static const u64 msi_bases[] = { MSI_IOVA_BASE, MSI_IOVA_BASE2 };
>  
>  	iommu_dma_get_resv_regions(dev, head);
> +
> +	/*
> +	 * Use the first msi_base that does not intersect with a platform
> +	 * reserved region. The SW MSI base selection is entirely arbitrary.
> +	 */
> +	for (int i = 0; i != ARRAY_SIZE(msi_bases); i++) {
> +		struct iommu_resv_region *region;
> +
> +		if (resv_region_intersects(msi_bases[i], MSI_IOVA_LENGTH, head))
> +			continue;
> +
> +		region = iommu_alloc_resv_region(msi_bases[i], MSI_IOVA_LENGTH, prot,
> +						 IOMMU_RESV_SW_MSI, GFP_KERNEL);
> +		if (!region)
> +			return;
> +
> +		list_add_tail(&region->list, head);
> +		return;
> +	}
>  }

I think this whole series looks pretty good, but I would suggest to
put this in a helper..

'iommu_set_sw_msi()' perhaps, must be called after iommu_dma_get_resv_regions()

Then maybe the constants can just be placed in the .c file inside the
helper function.

Jsaon