Re: [PATCH v2 0/3] arm-smmu: select suitable IOVA

[Jason Gunthorpe <jgg@ziepe.ca>]

On Wed, Apr 16, 2025 at 11:04:27AM -0700, Jacob Pan wrote:

> Per last discussion "SMMU driver have a list of potential addresses and
> select the first one that does not intersect with the non-working IOVA
> ranges.". If we don't know what the "non-working IOVA" is, how do we
> know it does not intersect the "potential addresses"?

I had understood from previous discussions that this platform is
properly creating IOMMU_RESV_RESERVED regions for the IOVA that
doesn't work. Otherwise everything is broken..

Presumably that happens through iommu_dma_get_resv_regions() calling
of_iommu_get_resv_regions() on a DT platform. There is a schema
describing how to do this, so platform firmware should be able to do it..

So the fix seems trivial enough to me:

diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
index b4c21aaed1266a..ebba18579151bc 100644
--- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
+++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
@@ -3562,17 +3562,29 @@ 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, 0x12340000 };
 
 	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 arbitary.
+	 */
+	for (i = 0; i != ARRAY_SIZE(msi_bases); i++) {
+		struct iommu_resv_region *region;
+
+		if (resv_intersects(msi_bases[i], MSI_IOVA_LENGTH))
+			continue;
+
+		region = iommu_alloc_resv_region(msi_bases[i], MSI_IOVA_LENGTH,
+						 IOMMU_WRITE | IOMMU_NOEXEC |
+							 IOMMU_MMIO,
+						 IOMMU_RESV_SW_MSI, GFP_KERNEL);
+		if (!region)
+			return;
+		list_add_tail(&region->list, head);
+		return;
+	}
 }
 
 static int arm_smmu_dev_enable_feature(struct device *dev,

Jason