[PATCH] iommu/riscv: Report cache coherency capability

[PATCH] iommu/riscv: Report cache coherency capability

[Andrew Jones]

VFIO and iommufd require IOMMU_CAP_CACHE_COHERENCY because they use
IOMMU_CACHE mappings and have no userspace cache-maintenance path.

RISC-V page tables preserve physical memory attributes for IOMMU_CACHE
mappings. Svpbmt marks mappings without IOMMU_CACHE as non-cacheable,
but preserving physical attributes cannot make a noncoherent device
path coherent.

Report the capability only for devices marked DMA coherent. This limits
userspace assignment to paths that can honor cacheable mappings.

Signed-off-by: Andrew Jones <andrew.jones@oss.qualcomm.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
---
 drivers/iommu/riscv/iommu.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/iommu/riscv/iommu.c b/drivers/iommu/riscv/iommu.c
index 22cccc69bd2c..161093b1d9c5 100644
--- a/drivers/iommu/riscv/iommu.c
+++ b/drivers/iommu/riscv/iommu.c
@@ -16,6 +16,7 @@
 #include <linux/acpi_rimt.h>
 #include <linux/compiler.h>
 #include <linux/crash_dump.h>
+#include <linux/dma-map-ops.h>
 #include <linux/init.h>
 #include <linux/iommu.h>
 #include <linux/iopoll.h>
@@ -1429,6 +1430,16 @@ static struct iommu_group *riscv_iommu_device_group(struct device *dev)
 	return generic_device_group(dev);
 }
 
+static bool riscv_iommu_capable(struct device *dev, enum iommu_cap cap)
+{
+	switch (cap) {
+	case IOMMU_CAP_CACHE_COHERENCY:
+		return dev_is_dma_coherent(dev);
+	default:
+		return false;
+	}
+}
+
 static int riscv_iommu_of_xlate(struct device *dev, const struct of_phandle_args *args)
 {
 	return iommu_fwspec_add_ids(dev, args->args, 1);
@@ -1520,6 +1531,7 @@ static void riscv_iommu_get_resv_regions(struct device *dev, struct list_head *h
 
 static const struct iommu_ops riscv_iommu_ops = {
 	.of_xlate = riscv_iommu_of_xlate,
+	.capable = riscv_iommu_capable,
 	.identity_domain = &riscv_iommu_identity_domain,
 	.blocked_domain = &riscv_iommu_blocking_domain,
 	.release_domain = &riscv_iommu_blocking_domain,
-- 
2.43.0

Re: [PATCH] iommu/riscv: Report cache coherency capability

[Jörg Rödel]

On Fri, Aug 21, 2026 at 01:21:00PM +0200, Andrew Jones wrote:
>  drivers/iommu/riscv/iommu.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)

Applied, thanks.