[PATCH v2 3/3] powerpc/mce: Handle memcpy_mcsafe

[Balbir Singh <bsingharora@gmail.com>]

Add a blocking notifier callback to be called in real-mode
on machine check exceptions for UE (ld/st) errors only.
The patch registers a callback on boot to be notified
of machine check exceptions and returns a NOTIFY_STOP when
a page of interest is seen as the source of the machine
check exception. This page of interest is a ZONE_DEVICE
page and hence for now, for memcpy_mcsafe to work, the page
needs to belong to ZONE_DEVICE and memcpy_mcsafe should be
used to access the memory.

The patch also modifies the NIP of the exception context
to go back to the fixup handler (in memcpy_mcsafe) and does
not print any error message as the error is treated as
returned via a return value and handled.

Signed-off-by: Balbir Singh <bsingharora@gmail.com>
---
 arch/powerpc/include/asm/mce.h |  3 +-
 arch/powerpc/kernel/mce.c      | 77 ++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 77 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/include/asm/mce.h b/arch/powerpc/include/asm/mce.h
index 3a1226e9b465..a76638e3e47e 100644
--- a/arch/powerpc/include/asm/mce.h
+++ b/arch/powerpc/include/asm/mce.h
@@ -125,7 +125,8 @@ struct machine_check_event {
 			enum MCE_UeErrorType ue_error_type:8;
 			uint8_t		effective_address_provided;
 			uint8_t		physical_address_provided;
-			uint8_t		reserved_1[5];
+			uint8_t		error_return;
+			uint8_t		reserved_1[4];
 			uint64_t	effective_address;
 			uint64_t	physical_address;
 			uint8_t		reserved_2[8];
diff --git a/arch/powerpc/kernel/mce.c b/arch/powerpc/kernel/mce.c
index efdd16a79075..b9e4881fa8c5 100644
--- a/arch/powerpc/kernel/mce.c
+++ b/arch/powerpc/kernel/mce.c
@@ -28,7 +28,9 @@
 #include <linux/percpu.h>
 #include <linux/export.h>
 #include <linux/irq_work.h>
+#include <linux/extable.h>
 
+#include <asm/extable.h>
 #include <asm/machdep.h>
 #include <asm/mce.h>
 
@@ -54,6 +56,52 @@ static struct irq_work mce_event_process_work = {
 
 DECLARE_WORK(mce_ue_event_work, machine_process_ue_event);
 
+static BLOCKING_NOTIFIER_HEAD(mce_notifier_list);
+
+int register_mce_notifier(struct notifier_block *nb)
+{
+	return blocking_notifier_chain_register(&mce_notifier_list, nb);
+}
+EXPORT_SYMBOL_GPL(register_mce_notifier);
+
+int unregister_mce_notifier(struct notifier_block *nb)
+{
+	return blocking_notifier_chain_unregister(&mce_notifier_list, nb);
+}
+EXPORT_SYMBOL_GPL(unregister_mce_notifier);
+
+
+static int check_memcpy_mcsafe(struct notifier_block *nb,
+			unsigned long val, void *data)
+{
+	/*
+	 * val contains the physical_address of the bad address
+	 */
+	unsigned long pfn = val >> PAGE_SHIFT;
+	struct page *page = realmode_pfn_to_page(pfn);
+	int rc = NOTIFY_DONE;
+
+	if (!page)
+		goto out;
+
+	if (is_zone_device_page(page))	/* for HMM and PMEM */
+		rc = NOTIFY_STOP;
+out:
+	return rc;
+}
+
+struct notifier_block memcpy_mcsafe_nb = {
+	.priority = 0,
+	.notifier_call = check_memcpy_mcsafe,
+};
+
+int  mce_mcsafe_register(void)
+{
+	register_mce_notifier(&memcpy_mcsafe_nb);
+	return 0;
+}
+arch_initcall(mce_mcsafe_register);
+
 static void mce_set_error_info(struct machine_check_event *mce,
 			       struct mce_error_info *mce_err)
 {
@@ -151,9 +199,31 @@ void save_mce_event(struct pt_regs *regs, long handled,
 		mce->u.ue_error.effective_address_provided = true;
 		mce->u.ue_error.effective_address = addr;
 		if (phys_addr != ULONG_MAX) {
+			int rc;
+			const struct exception_table_entry *entry;
+
+			/*
+			 * Once we have the physical address, we check to
+			 * see if the current nip has a fixup entry.
+			 * Having a fixup entry plus the notifier stating
+			 * that it can handle the exception is an indication
+			 * that we should return to the fixup entry and
+			 * return an error from there
+			 */
 			mce->u.ue_error.physical_address_provided = true;
 			mce->u.ue_error.physical_address = phys_addr;
-			machine_check_ue_event(mce);
+
+			rc = blocking_notifier_call_chain(&mce_notifier_list,
+							phys_addr, NULL);
+			if (rc & NOTIFY_STOP_MASK) {
+				entry = search_exception_tables(regs->nip);
+				if (entry != NULL) {
+					mce->u.ue_error.error_return = 1;
+					regs->nip = extable_fixup(entry);
+				} else
+					machine_check_ue_event(mce);
+			} else
+				machine_check_ue_event(mce);
 		}
 	}
 	return;
@@ -208,7 +278,6 @@ void release_mce_event(void)
 	get_mce_event(NULL, true);
 }
 
-
 /*
  * Queue up the MCE event which then can be handled later.
  */
@@ -239,6 +308,10 @@ void machine_check_queue_event(void)
 	if (!get_mce_event(&evt, MCE_EVENT_RELEASE))
 		return;
 
+	if (evt.error_type == MCE_ERROR_TYPE_UE &&
+			evt.u.ue_error.error_return == 1)
+		return;
+
 	index = __this_cpu_inc_return(mce_queue_count) - 1;
 	/* If queue is full, just return for now. */
 	if (index >= MAX_MC_EVT) {
-- 
2.13.6

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