[RFC PATCH] iommu/arm-smmu-v3: Allow nested attach for PCI bridges without vDEVICE

[Dmitry Malkin]

IOMMU groups can contain passed-through endpoints and host PCI bridges.
QEMU creates vDEVICEs for the endpoints, but not for bridges that are
not visible to the guest.

Since f0ea207ed781 ("iommu/arm-smmu-v3: Introduce struct
arm_smmu_vmaster"), group-wide translated-domain attachment therefore
fails when the first bridge is visited:

  pcieport 0009:00:00.0:
    missing vDEVICE association for translated nested domain:
    cfg=5 ret=-2

  vfio-pci 0009:03:00.0:
    VFIO_DEVICE_ATTACH_IOMMUFD_PT attach failed for pt_id 19: -ENOENT

The same configuration worked with Linux 6.14.

Allow PCI bridges without a vDEVICE to attach without a vmaster.
Endpoint devices still require a vDEVICE, preserving vSID handling for
invalidations and virtual events. Log when the bridge exception is used.

Fixes: f0ea207ed781 ("iommu/arm-smmu-v3: Introduce struct arm_smmu_vmaster")
Cc: stable at vger.kernel.org
Signed-off-by: Dmitry Malkin <dma at nebius.com>
---
 drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c
--- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c
+++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c
@@ -5,6 +5,8 @@
 
 #include <uapi/linux/iommufd.h>
 
+#include <linux/pci.h>
+
 #include "arm-smmu-v3.h"
 
 void *arm_smmu_hw_info(struct device *dev, u32 *length,
@@ -118,6 +120,17 @@ int arm_smmu_attach_prepare_vmaster(struct arm_smmu_attach_state *state,
 		if (cfg == STRTAB_STE_0_CFG_ABORT ||
 		    cfg == STRTAB_STE_0_CFG_BYPASS)
 			return 0;
+		/*
+		 * Group-wide domain attachment also visits host PCI bridges. Such a
+		 * bridge is not exposed to the VM and therefore has no virtual SID.
+		 */
+		if (dev_is_pci(state->master->dev) &&
+		    pci_is_bridge(to_pci_dev(state->master->dev))) {
+			dev_info_ratelimited(state->master->dev,
+					     "skipping vDEVICE requirement for translated nested domain: cfg=%u ret=%d\n",
+					     cfg, ret);
+			return 0;
+		}
 		return ret;
 	}
 
-- 
2.43.0