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iommu/riscv: Report cache coherency capability

PATCH Applied Andrew Jones 3 participants 3 messages Aug 21 – Nov 1

Subsystems (3)
RISC-V IOMMU RISC-V IOMMU
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Andrew Jones

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#0 lore raw

PATCH 1/1 iommu/riscv: Report cache coherency capability

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 <[email protected]>
Reviewed-by Jason Gunthorpe <[email protected]>
---
 drivers/iommu/riscv/iommu.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)
drivers/iommu/riscv/iommu.c +12 -0
index 22cccc69bd2c..161093b1d9c5 100644
@@ -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,
Signature · 1 line
2.43.0
maintainer Applied
#1 lore raw
Quoted 2 lines · Andrew Jones
 drivers/iommu/riscv/iommu.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

Applied, thanks.

CI Applied
#2 lore raw
In reply to Andrew Jones
This patch was applied to riscv/linux.git (fixes) by Joerg Roedel <[email protected]>:

Hello:

This patch was applied to riscv/linux.git (fixes)
by Joerg Roedel <[email protected]>:

Quoted 9 lines · On Fri, 21 Aug 2026 13:21:00 +0200 you wrote:

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.

[...]

Here is the summary with links:
- iommu/riscv: Report cache coherency capability

    https://git.kernel.org/riscv/c/3069e8f3e1e9

You are awesome, thank you!

Signature · 2 lines
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