[RFC PATCH v2 08/10] iommu/arm-smmu-v3: Avoid suspend when user owns DMA

[Pranjal Shrivastava <praan@google.com>]

Suspending the smmu is unsafe if any attached devices belong
to iommu groups claimed by user-space (via VFIO etc.), as a DMA may
be ongoing or initiated at any time. The smmu must remain powered ON to
handle potential DMA requests originating from the user-space context.

Introduce a helper `arm_smmu_suspend_is_safe()` using the previously
added `insecure_attachments` list. If the check fails (list is not empty),
defer the suspend ensuring the SMMU remains active while it's potentially
needed by user-space for DMA.

Signed-off-by: Pranjal Shrivastava <praan@google.com>
---
 drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 18 ++++++++++++++++++
 drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h |  2 +-
 2 files changed, 19 insertions(+), 1 deletion(-)

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 2b51665d5ca6..7099c0cbf5fe 100644
--- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
+++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
@@ -5040,11 +5040,29 @@ static void arm_smmu_device_shutdown(struct platform_device *pdev)
 	arm_smmu_device_disable(smmu);
 }
 
+static bool arm_smmu_suspend_is_safe(struct arm_smmu_device *smmu)
+{
+	unsigned long flags;
+	bool is_empty;
+
+	spin_lock_irqsave(&smmu->attach_lock, flags);
+	is_empty = list_empty(&smmu->insecure_attachments);
+	spin_unlock_irqrestore(&smmu->attach_lock, flags);
+
+	return is_empty;
+}
+
 static int __maybe_unused arm_smmu_runtime_suspend(struct device *dev)
 {
 	struct arm_smmu_device *smmu = dev_get_drvdata(dev);
 	int ret;
 
+	/* Refuse to suspend if user-space controls DMA */
+	if (!arm_smmu_suspend_is_safe(smmu)) {
+		dev_err(smmu->dev, "Suspend deferred due to unsafe DMA owners\n");
+		return -EAGAIN;
+	}
+
 	/*
 	 * Since suspend is invoked when all clients have been suspended,
 	 * we don't expect more cmds or events to be added to the queues.
diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h
index 5ab60ae7ff91..28fddc07ab49 100644
--- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h
+++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h
@@ -796,7 +796,7 @@ struct arm_smmu_device {
 	struct rb_root			streams;
 	struct mutex			streams_mutex;
 
-	/* Insecure Attach handles */
+	/* Insecure attach handles */
 	struct list_head		insecure_attachments;
 	/* Lock for the list */
 	spinlock_t			attach_lock;
-- 
2.49.0.805.g082f7c87e0-goog