linux-kernel - [RFC PATCH v2 15/32] iommu: Add API to keep track of iommu domain attachments

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Message-ID: <20251202230303.1017519-16-skhawaja@google.com>
Date: Tue,  2 Dec 2025 23:02:45 +0000
From: Samiullah Khawaja <skhawaja@...gle.com>
To: David Woodhouse <dwmw2@...radead.org>, Lu Baolu <baolu.lu@...ux.intel.com>, 
	Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>, 
	Pasha Tatashin <pasha.tatashin@...een.com>, Jason Gunthorpe <jgg@...pe.ca>, iommu@...ts.linux.dev
Cc: Samiullah Khawaja <skhawaja@...gle.com>, Robin Murphy <robin.murphy@....com>, 
	Pratyush Yadav <pratyush@...nel.org>, Kevin Tian <kevin.tian@...el.com>, 
	Alex Williamson <alex@...zbot.org>, linux-kernel@...r.kernel.org, 
	Saeed Mahameed <saeedm@...dia.com>, Adithya Jayachandran <ajayachandra@...dia.com>, 
	Parav Pandit <parav@...dia.com>, Leon Romanovsky <leonro@...dia.com>, William Tu <witu@...dia.com>, 
	Vipin Sharma <vipinsh@...gle.com>, dmatlack@...gle.com, YiFei Zhu <zhuyifei@...gle.com>, 
	Chris Li <chrisl@...nel.org>, praan@...gle.com
Subject: [RFC PATCH v2 15/32] iommu: Add API to keep track of iommu domain attachments

Keep track of iommu domain attachments by incrementing and decremeting a
counter when a device is attached and detached to a domain.

Signed-off-by: Samiullah Khawaja <skhawaja@...gle.com>
---
 drivers/iommu/iommu.c | 18 ++++++++++++++++++
 include/linux/iommu.h |  3 +++
 2 files changed, 21 insertions(+)

diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 2ca990dfbb88..a70898d11959 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -2041,6 +2041,7 @@ static void iommu_domain_init(struct iommu_domain *domain, unsigned int type,
 {
 	domain->type = type;
 	domain->owner = ops;
+	atomic_set(&domain->attach_count, 0);
 	if (!domain->ops)
 		domain->ops = ops->default_domain_ops;
 }
@@ -2093,8 +2094,20 @@ struct iommu_domain *iommu_paging_domain_alloc_flags(struct device *dev,
 }
 EXPORT_SYMBOL_GPL(iommu_paging_domain_alloc_flags);
 
+bool iommu_domain_has_attachments(struct iommu_domain *domain)
+{
+	return atomic_read(&domain->attach_count) != 0;
+}
+EXPORT_SYMBOL_GPL(iommu_domain_has_attachments);
+
 void iommu_domain_free(struct iommu_domain *domain)
 {
+	if (WARN_ON_ONCE(iommu_domain_has_attachments(domain))) {
+		pr_err("Attempt to free an iommu_domain that has attachments: %d\n",
+		       atomic_read(&domain->attach_count));
+		return;
+	}
+
 	switch (domain->cookie_type) {
 	case IOMMU_COOKIE_DMA_IOVA:
 		iommu_put_dma_cookie(domain);
@@ -2140,6 +2153,11 @@ static int __iommu_attach_device(struct iommu_domain *domain,
 	ret = domain->ops->attach_dev(domain, dev, old);
 	if (ret)
 		return ret;
+
+	atomic_inc(&domain->attach_count);
+	if (old)
+		atomic_dec(&old->attach_count);
+
 	dev->iommu->attach_deferred = 0;
 	trace_attach_device_to_domain(dev);
 	return 0;
diff --git a/include/linux/iommu.h b/include/linux/iommu.h
index 17e1f3c29958..387cbe20c83b 100644
--- a/include/linux/iommu.h
+++ b/include/linux/iommu.h
@@ -14,6 +14,7 @@
 #include <linux/err.h>
 #include <linux/of.h>
 #include <linux/iova_bitmap.h>
+#include <linux/atomic.h>
 #include <linux/kho/abi/iommu.h>
 #include <uapi/linux/iommufd.h>
 
@@ -250,6 +251,7 @@ struct iommu_domain {
 		};
 	};
 
+	atomic_t attach_count;
 #ifdef CONFIG_LIVEUPDATE
 	struct iommu_domain_ser *preserved_state;
 #endif
@@ -917,6 +919,7 @@ static inline struct iommu_domain *iommu_paging_domain_alloc(struct device *dev)
 {
 	return iommu_paging_domain_alloc_flags(dev, 0);
 }
+extern bool iommu_domain_has_attachments(struct iommu_domain *domain);
 extern void iommu_domain_free(struct iommu_domain *domain);
 extern int iommu_attach_device(struct iommu_domain *domain,
 			       struct device *dev);
-- 
2.52.0.158.g65b55ccf14-goog