To support serialising persistent iommufd objects to KHO, and to be able
to restore the persisted data it is necessary to have a serialise hook
on the KHO active path as well as a deserialise hook on module init.

This commit adds those hooks and the new serialise.c file which will
hold the logic here; for now it's just empty functions.
---
 drivers/iommu/iommufd/Makefile          |  1 +
 drivers/iommu/iommufd/iommufd_private.h | 22 +++++++++++++++++++++
 drivers/iommu/iommufd/main.c            | 24 ++++++++++++++++++++++-
 drivers/iommu/iommufd/serialise.c       | 26 +++++++++++++++++++++++++
 4 files changed, 72 insertions(+), 1 deletion(-)
 create mode 100644 drivers/iommu/iommufd/serialise.c

diff --git a/drivers/iommu/iommufd/Makefile b/drivers/iommu/iommufd/Makefile
index cf4605962bea..80bc775c170d 100644
--- a/drivers/iommu/iommufd/Makefile
+++ b/drivers/iommu/iommufd/Makefile
@@ -13,3 +13,4 @@ iommufd-$(CONFIG_IOMMUFD_TEST) += selftest.o
 
 obj-$(CONFIG_IOMMUFD) += iommufd.o
 obj-$(CONFIG_IOMMUFD_DRIVER) += iova_bitmap.o
+obj-$(CONFIG_KEXEC_KHO) += serialise.o
diff --git a/drivers/iommu/iommufd/iommufd_private.h b/drivers/iommu/iommufd/iommufd_private.h
index b23f7766066c..a26728646a22 100644
--- a/drivers/iommu/iommufd/iommufd_private.h
+++ b/drivers/iommu/iommufd/iommufd_private.h
@@ -497,6 +497,28 @@ static inline void iommufd_hwpt_detach_device(struct iommufd_hw_pagetable *hwpt,
 	iommu_detach_group(hwpt->domain, idev->igroup->group);
 }
 
+/*
+ * Serialise is invoked as a callback by KHO when changing KHO active state,
+ * it stores current iommufd state into KHO's persistent store.
+ * Deserialise is run by the iommufd module when loaded to re-hydrate state
+ * carried across from the previous kernel.
+ */
+#ifdef CONFIG_KEXEC_KHO
+int iommufd_serialise_kho(struct notifier_block *self, unsigned long cmd,
+			  void *fdt);
+int __init iommufd_deserialise_kho(void);
+#else
+int iommufd_serialise_kho(struct notifier_block *self, unsigned long cmd,
+			  void *fdt)
+{
+	return 0;
+}
+int __init iommufd_deserialise_kho(void)
+{
+	return 0;
+}
+#endif
+
 static inline int iommufd_hwpt_replace_device(struct iommufd_device *idev,
 					      struct iommufd_hw_pagetable *hwpt,
 					      struct iommufd_hw_pagetable *old)
diff --git a/drivers/iommu/iommufd/main.c b/drivers/iommu/iommufd/main.c
index 6708ad629b1e..fa4f0fe336ad 100644
--- a/drivers/iommu/iommufd/main.c
+++ b/drivers/iommu/iommufd/main.c
@@ -10,6 +10,7 @@
 
 #include <linux/file.h>
 #include <linux/fs.h>
+#include <linux/kexec.h>
 #include <linux/module.h>
 #include <linux/slab.h>
 #include <linux/miscdevice.h>
@@ -590,6 +591,10 @@ static struct miscdevice vfio_misc_dev = {
 	.mode = 0666,
 };
 
+static struct notifier_block serialise_kho_nb = {
+	.notifier_call = iommufd_serialise_kho,
+};
+
 static int __init iommufd_init(void)
 {
 	int ret;
@@ -603,11 +608,26 @@ static int __init iommufd_init(void)
 		if (ret)
 			goto err_misc;
 	}
+
+	if (IS_ENABLED(CONFIG_KEXEC_KHO)) {
+		ret = register_kho_notifier(&serialise_kho_nb);
+		if (ret)
+			goto err_vfio_misc;
+	}
+
+	ret = iommufd_deserialise_kho();
+	if (ret)
+		goto err_kho;
+
 	ret = iommufd_test_init();
+
 	if (ret)
-		goto err_vfio_misc;
+		goto err_kho;
 	return 0;
 
+err_kho:
+	if (IS_ENABLED(CONFIG_KEXEC_KHO))
+		unregister_kho_notifier(&serialise_kho_nb);
 err_vfio_misc:
 	if (IS_ENABLED(CONFIG_IOMMUFD_VFIO_CONTAINER))
 		misc_deregister(&vfio_misc_dev);
@@ -621,6 +641,8 @@ static void __exit iommufd_exit(void)
 	iommufd_test_exit();
 	if (IS_ENABLED(CONFIG_IOMMUFD_VFIO_CONTAINER))
 		misc_deregister(&vfio_misc_dev);
+	if (IS_ENABLED(CONFIG_FTRACE_KHO))
+		unregister_kho_notifier(&serialise_kho_nb);
 	misc_deregister(&iommu_misc_dev);
 }
 
diff --git a/drivers/iommu/iommufd/serialise.c b/drivers/iommu/iommufd/serialise.c
new file mode 100644
index 000000000000..6e8bcc384771
--- /dev/null
+++ b/drivers/iommu/iommufd/serialise.c
@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#include <linux/kexec.h>
+#include "iommufd_private.h"
+
+int iommufd_serialise_kho(struct notifier_block *self, unsigned long cmd,
+			  void *fdt)
+{
+	pr_info("would serialise here\n");
+	switch (cmd) {
+	case KEXEC_KHO_ABORT:
+		/* Would do serialise rollback here. */
+		return NOTIFY_DONE;
+	case KEXEC_KHO_DUMP:
+		/* Would do serialise here. */
+		return NOTIFY_DONE;
+	default:
+		return NOTIFY_BAD;
+	}
+}
+
+int __init iommufd_deserialise_kho(void)
+{
+	pr_info("would deserialise here\n");
+	return 0;
+}
-- 
2.34.1