[PATCH 5/5] x86/microcode: Handle NMI's during microcode update.

[Ashok Raj]

Microcode updates need a guarantee that the thread sibling that is waiting
for the update to finish on the primary core will not execute any
instructions until the update is complete. This is required to guarantee
any MSR or instruction that's being patched will be executed before the
update is complete.

After the stop_machine() rendezvous, an NMI handler is registered. If an
NMI were to happen while the microcode update is not complete, the
secondary thread will spin until the ucode update state is cleared.

Couple of choices discussed are:

1. Rendezvous inside the NMI handler, and also perform the update from
   within the handler. This seemed too risky and might cause instability
   with the races that we would need to solve. This would be a difficult
   choice.
2. Thomas (tglx) suggested that we could look into masking all the LVT
   originating NMI's. Such as LINT1, Perf control LVT entries and such.
   Since we are in the rendezvous loop, we don't need to worry about any
   NMI IPI's generated by the OS.

   The one we didn't have any control over is the ACPI mechanism of sending
   notifications to kernel for Firmware First Processing (FFM). Apparently
   it seems there is a PCH register that BIOS in SMI would write to
   generate such an interrupt (ACPI GHES).
3. This is a simpler option. OS registers an NMI handler and doesn't do any
   NMI rendezvous dance. But if an NMI were to happen, we check if any of
   the CPUs thread siblings have an update in progress. Only those CPUs
   would take an NMI. The thread performing the wrmsr() will only take an
   NMI after the completion of the wrmsr 0x79 flow.

Signed-off-by: Ashok Raj <ashok.raj@xxxxxxxxx>
---
 arch/x86/kernel/cpu/microcode/core.c | 88 +++++++++++++++++++++++++++-
 1 file changed, 85 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index d24e1c754c27..ec10fa2db8b1 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -40,6 +40,7 @@
 #include <asm/cmdline.h>
 #include <asm/setup.h>
 #include <asm/mce.h>
+#include <asm/nmi.h>
 
 #define DRIVER_VERSION	"2.2"
 
@@ -411,6 +412,10 @@ static int check_online_cpus(void)
 
 static atomic_t late_cpus_in;
 static atomic_t late_cpus_out;
+static atomic_t nmi_cpus;
+static atomic_t nmi_timeouts;
+
+static struct cpumask cpus_in_wait;
 
 static int __wait_for_cpus(atomic_t *t, long long timeout)
 {
@@ -433,6 +438,53 @@ static int __wait_for_cpus(atomic_t *t, long long timeout)
 	return 0;
 }
 
+static int ucode_nmi_cb(unsigned int val, struct pt_regs *regs)
+{
+	int cpu = smp_processor_id();
+	int timeout = 100 * NSEC_PER_USEC;
+
+	atomic_inc(&nmi_cpus);
+	if (!cpumask_test_cpu(cpu, &cpus_in_wait))
+		return NMI_DONE;
+
+	while (timeout < NSEC_PER_USEC) {
+		if (timeout < NSEC_PER_USEC) {
+			atomic_inc(&nmi_timeouts);
+			break;
+		}
+		ndelay(SPINUNIT);
+		timeout -= SPINUNIT;
+		touch_nmi_watchdog();
+		if (!cpumask_test_cpu(cpu, &cpus_in_wait))
+			break;
+	}
+	return NMI_HANDLED;
+}
+
+static void set_nmi_cpus(struct cpumask *wait_mask)
+{
+	int first_cpu, wait_cpu, cpu = smp_processor_id();
+
+	first_cpu = cpumask_first(topology_sibling_cpumask(cpu));
+	for_each_cpu(wait_cpu, topology_sibling_cpumask(cpu)) {
+		if (wait_cpu == first_cpu)
+			continue;
+		cpumask_set_cpu(wait_cpu, wait_mask);
+	}
+}
+
+static void clear_nmi_cpus(struct cpumask *wait_mask)
+{
+	int first_cpu, wait_cpu, cpu = smp_processor_id();
+
+	first_cpu = cpumask_first(topology_sibling_cpumask(cpu));
+	for_each_cpu(wait_cpu, topology_sibling_cpumask(cpu)) {
+		if (wait_cpu == first_cpu)
+			continue;
+		cpumask_clear_cpu(wait_cpu, wait_mask);
+	}
+}
+
 /*
  * Returns:
  * < 0 - on error
@@ -440,7 +492,7 @@ static int __wait_for_cpus(atomic_t *t, long long timeout)
  */
 static int __reload_late(void *info)
 {
-	int cpu = smp_processor_id();
+	int first_cpu, cpu = smp_processor_id();
 	enum ucode_state err;
 	int ret = 0;
 
@@ -459,6 +511,7 @@ static int __reload_late(void *info)
 	 * the platform is taken to reset predictively.
 	 */
 	mce_set_mcip();
+
 	/*
 	 * On an SMT system, it suffices to load the microcode on one sibling of
 	 * the core because the microcode engine is shared between the threads.
@@ -466,9 +519,17 @@ static int __reload_late(void *info)
 	 * loading attempts happen on multiple threads of an SMT core. See
 	 * below.
 	 */
+	first_cpu = cpumask_first(topology_sibling_cpumask(cpu));
 
-	if (cpumask_first(topology_sibling_cpumask(cpu)) == cpu)
+	/*
+	 * Set the CPUs that we should hold in NMI until the primary has
+	 * completed the microcode update.
+	 */
+	if (first_cpu == cpu) {
+		set_nmi_cpus(&cpus_in_wait);
 		apply_microcode_local(&err);
+		clear_nmi_cpus(&cpus_in_wait);
+	}
 	else
 		goto wait_for_siblings;
 
@@ -502,20 +563,41 @@ static int __reload_late(void *info)
  */
 static int microcode_reload_late(void)
 {
-	int ret;
+	int ret = 0;
 
 	pr_err("Attempting late microcode loading - it is dangerous and taints the kernel.\n");
 	pr_err("You should switch to early loading, if possible.\n");
 
 	atomic_set(&late_cpus_in,  0);
 	atomic_set(&late_cpus_out, 0);
+	atomic_set(&nmi_cpus, 0);
+	atomic_set(&nmi_timeouts, 0);
+	cpumask_clear(&cpus_in_wait);
+
+	ret = register_nmi_handler(NMI_LOCAL, ucode_nmi_cb, NMI_FLAG_FIRST,
+				   "ucode_nmi");
+	if (ret) {
+		pr_err("Unable to register NMI handler\n");
+		goto done;
+	}
 
 	ret = stop_machine_cpuslocked(__reload_late, NULL, cpu_online_mask);
 	if (ret == 0)
 		microcode_check();
 
+	unregister_nmi_handler(NMI_LOCAL, "ucode_nmi");
+
+	if (atomic_read(&nmi_cpus))
+		pr_info("%d CPUs entered NMI while microcode update"
+			"in progress\n", atomic_read(&nmi_cpus));
+
+	if (atomic_read(&nmi_timeouts))
+		pr_err("Some CPUs [%d] entered NMI and timedout waiting for its"
+		       " mask to be cleared\n", atomic_read(&nmi_timeouts));
+
 	pr_info("Reload completed, microcode revision: 0x%x\n", boot_cpu_data.microcode);
 
+done:
 	return ret;
 }
 
-- 
2.32.0