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Re: [PATCH v3 23/23] doc/arm: vIOMMU design document
- To: Milan Djokic <milan_djokic@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: abhash <a-kumar2@xxxxxx>
- Date: Tue, 28 Jul 2026 16:28:49 +0530
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+Performance Impact
+==================
+
+With iommu stage-1 and nested translation inclusion, performance
+overhead is introduced comparing to existing,
+stage-2 only usage in Xen. Once mappings are established, translations
+should not introduce significant overhead.
+Emulated paths may introduce moderate overhead, primarily affecting
+device initialization and event/command handling.
+Testing is performed on Renesas R-Car platform.
Hi Milan,
I tested this series on a TI platform. I was not able to get the DMA
test inside a DOM1 guest
to work with nested virtualization.
On the top level, my changes are:
- Enabled CONFIG_VIRTUAL_IOMMU and CONFIG_VIRTUAL_ARM_SMMU_V3 in Xen
- Added viommu=true in xen-bootargs
- Added a "smmu" node in my DOM1 passthrough node, so that it can be
replaced by the viommu.
- Added "iommus" for my DMA controller.
I can see that the smmuv3 is probed successfully inside of DOM1 and my
dma-controller is added to the iommu group.
""
[ 0.161594] arm-smmu-v3 4040000.iommu: probing device
[ 0.162067] arm-smmu-v3 4040000.iommu: ias 48-bit, oas 48-bit
(features 0x0000020d)
[ 0.162993] arm-smmu-v3 4040000.iommu: allocated 65536 entries for cmdq
[ 0.163730] arm-smmu-v3 4040000.iommu: allocated 32768 entries for evtq
[ 0.164281] arm_smmu_device_reset
[ 0.164559] ARM_SMMU_GBPA reg = 0
[ 0.164924] smmu->evtq.q.q_base = 45a0000f
[ 0.165609] platform 30100000.dma-controller: Adding to iommu group 0
""
Did you have any changes in the kernel tree as well?
Could you provide the testing setup that you used?
Best Regards,
Abhash
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