[PATCH v4 3/3] mshv: unmap debugfs stats pages on kexec

[Jork Loeser <jloeser@linux.microsoft.com>]

On L1VH, debugfs stats pages are overlay pages: the kernel allocates
them and registers the GPAs with the hypervisor via
HVCALL_MAP_STATS_PAGE2. These overlay mappings persist in the
hypervisor across kexec. If the kexec'd kernel reuses those physical
pages, the hypervisor's overlay semantics cause a machine check
exception.

Fix this by calling mshv_debugfs_exit() from the reboot notifier,
which issues HVCALL_UNMAP_STATS_PAGE for each mapped stats page before
kexec. This releases the overlay bindings so the physical pages can be
safely reused. Guard mshv_debugfs_exit() against being called when
init failed.

Signed-off-by: Jork Loeser <jloeser@linux.microsoft.com>
---
 drivers/hv/mshv_debugfs.c | 7 ++++++-
 drivers/hv/mshv_synic.c   | 1 +
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/hv/mshv_debugfs.c b/drivers/hv/mshv_debugfs.c
index 418b6dc8f3c2..3c3e02237ae9 100644
--- a/drivers/hv/mshv_debugfs.c
+++ b/drivers/hv/mshv_debugfs.c
@@ -674,8 +674,10 @@ int __init mshv_debugfs_init(void)
 
 	mshv_debugfs = debugfs_create_dir("mshv", NULL);
 	if (IS_ERR(mshv_debugfs)) {
+		err = PTR_ERR(mshv_debugfs);
+		mshv_debugfs = NULL;
 		pr_err("%s: failed to create debugfs directory\n", __func__);
-		return PTR_ERR(mshv_debugfs);
+		return err;
 	}
 
 	if (hv_root_partition()) {
@@ -710,6 +712,9 @@ int __init mshv_debugfs_init(void)
 
 void mshv_debugfs_exit(void)
 {
+	if (!mshv_debugfs)
+		return;
+
 	mshv_debugfs_parent_partition_remove();
 
 	if (hv_root_partition()) {
diff --git a/drivers/hv/mshv_synic.c b/drivers/hv/mshv_synic.c
index 978a1cace341..88170ce6b83f 100644
--- a/drivers/hv/mshv_synic.c
+++ b/drivers/hv/mshv_synic.c
@@ -723,6 +723,7 @@ mshv_unregister_doorbell(u64 partition_id, int doorbell_portid)
 static int mshv_synic_reboot_notify(struct notifier_block *nb,
 			      unsigned long code, void *unused)
 {
+	mshv_debugfs_exit();
 	cpuhp_remove_state(synic_cpuhp_online);
 	return 0;
 }
-- 
2.43.0