[RFC Patch v1 4/4] arm64: kgdb: Round up cpus using IPI_CALL_NMI_FUNC

[Sumit Garg]

arm64 platforms with GICv3 or later supports pseudo NMIs which can be
leveraged to round up CPUs which are stuck in hard lockup state with
interrupts disabled that wouldn't be possible with a normal IPI.

So instead switch to round up CPUs using IPI_CALL_NMI_FUNC. And in
case a particular arm64 platform doesn't supports pseudo NMIs,
IPI_CALL_NMI_FUNC will act as a normal IPI which maintains existing
kgdb functionality.

Also, one thing to note here is that with CPUs running in NMI context,
kernel has special handling for printk() which involves CPU specific
buffers and defering printk() until exit from NMI context. But with kgdb
we don't want to defer printk() especially backtrace on corresponding
CPUs. So switch to normal printk() context instead prior to entering
kgdb context.

Signed-off-by: Sumit Garg <sumit.garg@xxxxxxxxxx>
---
 arch/arm64/kernel/kgdb.c | 15 +++++++++++++++
 arch/arm64/kernel/smp.c  | 17 ++++++++++++++---
 2 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c
index 4311992..0851ead 100644
--- a/arch/arm64/kernel/kgdb.c
+++ b/arch/arm64/kernel/kgdb.c
@@ -14,6 +14,7 @@
 #include <linux/kgdb.h>
 #include <linux/kprobes.h>
 #include <linux/sched/task_stack.h>
+#include <linux/smp.h>
 
 #include <asm/debug-monitors.h>
 #include <asm/insn.h>
@@ -353,3 +354,17 @@ int kgdb_arch_remove_breakpoint(struct kgdb_bkpt *bpt)
 	return aarch64_insn_write((void *)bpt->bpt_addr,
 			*(u32 *)bpt->saved_instr);
 }
+
+#ifdef CONFIG_SMP
+void kgdb_roundup_cpus(void)
+{
+	struct cpumask mask;
+
+	cpumask_copy(&mask, cpu_online_mask);
+	cpumask_clear_cpu(raw_smp_processor_id(), &mask);
+	if (cpumask_empty(&mask))
+		return;
+
+	arch_send_call_nmi_func_ipi_mask(&mask);
+}
+#endif
diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index 27c8ee1..c7158f6e8 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -31,6 +31,7 @@
 #include <linux/of.h>
 #include <linux/irq_work.h>
 #include <linux/kexec.h>
+#include <linux/kgdb.h>
 #include <linux/kvm_host.h>
 
 #include <asm/alternative.h>
@@ -976,9 +977,19 @@ void handle_IPI(int ipinr, struct pt_regs *regs)
 		/* Handle it as a normal interrupt if not in NMI context */
 		if (!in_nmi())
 			irq_enter();
-
-		/* nop, IPI handlers for special features can be added here. */
-
+#ifdef CONFIG_KGDB
+		if (atomic_read(&kgdb_active) != -1) {
+			/*
+			 * For kgdb to work properly, we need printk to operate
+			 * in normal context.
+			 */
+			if (in_nmi())
+				printk_nmi_exit();
+			kgdb_nmicallback(raw_smp_processor_id(), regs);
+			if (in_nmi())
+				printk_nmi_enter();
+		}
+#endif
 		if (!in_nmi())
 			irq_exit();
 		break;
-- 
2.7.4