arm64: kgdb: Implement kgdb_roundup_cpus() to enable pseudo-NMI roundup

Up until now we've been using the generic (weak) implementation for
kgdb_roundup_cpus() when using kgdb on arm64. Let's move to a custom
one. The advantage here is that, when pseudo-NMI is enabled on a
device, we'll be able to round up CPUs using pseudo-NMI. This allows
us to debug CPUs that are stuck with interrupts disabled. If
pseudo-NMIs are not enabled then we'll fallback to just using an IPI,
which is still slightly better than the generic implementation since
it avoids the potential situation described in the generic
kgdb_call_nmi_hook().

Co-developed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Chen-Yu Tsai <wenst@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20230906090246.v13.6.I2ef26d1b3bfbed2d10a281942b0da7d9854de05e@changeid
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/smp.c

@@ -32,6 +32,7 @@
 #include <linux/irq_work.h>
 #include <linux/kernel_stat.h>
 #include <linux/kexec.h>
+#include <linux/kgdb.h>
 #include <linux/kvm_host.h>
 #include <linux/nmi.h>
 
@@ -79,6 +80,7 @@ enum ipi_msg_type {
 	 * with trace_ipi_*
 	 */
 	IPI_CPU_BACKTRACE = NR_IPI,
+	IPI_KGDB_ROUNDUP,
 	MAX_IPI
 };
 
@@ -868,6 +870,22 @@ void arch_trigger_cpumask_backtrace(const cpumask_t *mask, int exclude_cpu)
 	nmi_trigger_cpumask_backtrace(mask, exclude_cpu, arm64_backtrace_ipi);
 }
 
+#ifdef CONFIG_KGDB
+void kgdb_roundup_cpus(void)
+{
+	int this_cpu = raw_smp_processor_id();
+	int cpu;
+
+	for_each_online_cpu(cpu) {
+		/* No need to roundup ourselves */
+		if (cpu == this_cpu)
+			continue;
+
+		__ipi_send_single(ipi_desc[IPI_KGDB_ROUNDUP], cpu);
+	}
+}
+#endif
+
 /*
  * Main handler for inter-processor interrupts
  */
@@ -919,6 +937,10 @@ static void do_handle_IPI(int ipinr)
 		nmi_cpu_backtrace(get_irq_regs());
 		break;
 
+	case IPI_KGDB_ROUNDUP:
+		kgdb_nmicallback(cpu, get_irq_regs());
+		break;
+
 	default:
 		pr_crit("CPU%u: Unknown IPI message 0x%x\n", cpu, ipinr);
 		break;
@@ -949,6 +971,7 @@ static bool ipi_should_be_nmi(enum ipi_msg_type ipi)
 	case IPI_CPU_STOP:
 	case IPI_CPU_CRASH_STOP:
 	case IPI_CPU_BACKTRACE:
+	case IPI_KGDB_ROUNDUP:
 		return true;
 	default:
 		return false;