linux-kernel - [PATCH v5 14/27] iommu/arm-smmu-v3: Support probing KVM emulated devices

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Message-ID: <20251117184815.1027271-15-smostafa@google.com>
Date: Mon, 17 Nov 2025 18:48:01 +0000
From: Mostafa Saleh <smostafa@...gle.com>
To: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	kvmarm@...ts.linux.dev, iommu@...ts.linux.dev
Cc: catalin.marinas@....com, will@...nel.org, maz@...nel.org, 
	oliver.upton@...ux.dev, joey.gouly@....com, suzuki.poulose@....com, 
	yuzenghui@...wei.com, joro@...tes.org, jean-philippe@...aro.org, jgg@...pe.ca, 
	praan@...gle.com, danielmentz@...gle.com, mark.rutland@....com, 
	qperret@...gle.com, tabba@...gle.com, Mostafa Saleh <smostafa@...gle.com>
Subject: [PATCH v5 14/27] iommu/arm-smmu-v3: Support probing KVM emulated devices

When KVM runs in protected mode, and CONFIG_ARM_SMMU_V3_PKVM
is enabled, it will manage the SMMUv3 HW using trap and emulate
and present emulated SMMUs to the host kernel.

In that case, those SMMUs will be on the aux bus, so make it
possibly to the driver to probe those devices.
Otherwise, everything else is the same as the KVM emulation
complies with the architecutre, so the driver doesn't need
to be modified.

Suggested-by: Jason Gunthorpe <jgg@...pe.ca>
Signed-off-by: Mostafa Saleh <smostafa@...gle.com>
---
 drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 58 +++++++++++++++++++++
 1 file changed, 58 insertions(+)

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 7b1bd0658910..851d47bedae6 100644
--- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
+++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
@@ -11,6 +11,7 @@
 
 #include <linux/acpi.h>
 #include <linux/acpi_iort.h>
+#include <linux/auxiliary_bus.h>
 #include <linux/bitops.h>
 #include <linux/crash_dump.h>
 #include <linux/delay.h>
@@ -4604,6 +4605,63 @@ static struct platform_driver arm_smmu_driver = {
 module_driver(arm_smmu_driver, platform_driver_register,
 	      arm_smmu_driver_unregister);
 
+#ifdef CONFIG_ARM_SMMU_V3_PKVM
+/*
+ * Now we have 2 devices, the aux device bound to this driver, and pdev
+ * which is the physical platform device.
+ * This part is a bit hairy but it works due to the fact that
+ * CONFIG_ARM_SMMU_V3_PKVM forces both drivers to be built in.
+ * The struct device for the SMMU is used in the following cases:
+ * 1) Printing using dev_*()
+ * 2) DMA memory alloc (dmam_alloc_coherent, devm_*)
+ * 3) Requesting resources (iomem, sysfs)
+ * 4) Probing firmware info (of_node, fwnode...)
+ * 5) Dealing with abstracted HW resources (irqs, MSIs, RPM)
+ * 6) Saving/reading driver data
+ * For point 4) and 5) we must use the platform device.
+ * For, 1) pdev is better for debuggability.
+ * For 2), 3), 6) it's better to use the bound device.
+ * However that doesn't really work:
+ * For 2) The DMA allocation using the aux device will fail, as
+ * we need to setup some device DMA attrs (mask), to match the
+ * platform.
+ * For 6) Some contexts from the pdev as MSI, it needs to use the
+ * drvdata.
+ * Based on the following:
+ * 1- Both drivers must be built-in to enable this (enforced by Kconfig),
+ *    which means that none of them can be removed.
+ * 2- The KVM driver doesn't do anythng at runtime and doesn't use drvdata.
+ * We can keep the driver simple and to claim the platform device in all cases.
+ */
+static int arm_smmu_device_probe_emu(struct auxiliary_device *auxdev,
+				     const struct auxiliary_device_id *id)
+{
+	struct device *parent = auxdev->dev.parent;
+
+	dev_info(&auxdev->dev, "Probing from %s\n", dev_name(parent));
+	return arm_smmu_device_probe(to_platform_device(parent));
+}
+
+static void arm_smmu_device_shutdown_emu(struct auxiliary_device *auxdev)
+{
+	arm_smmu_device_shutdown(to_platform_device(auxdev->dev.parent));
+}
+
+const struct auxiliary_device_id arm_smmu_aux_table[] = {
+	{ .name = "protected_kvm.smmu_v3_emu" },
+	{ },
+};
+
+struct auxiliary_driver arm_smmu_driver_emu = {
+	.name = "arm-smmu-v3-emu",
+	.id_table = arm_smmu_aux_table,
+	.probe = arm_smmu_device_probe_emu,
+	.shutdown = arm_smmu_device_shutdown_emu,
+};
+
+module_auxiliary_driver(arm_smmu_driver_emu);
+#endif
+
 MODULE_DESCRIPTION("IOMMU API for ARM architected SMMUv3 implementations");
 MODULE_AUTHOR("Will Deacon <will@...nel.org>");
 MODULE_ALIAS("platform:arm-smmu-v3");
-- 
2.52.0.rc1.455.g30608eb744-goog