[PATCH v2 4/4] 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.
   	1.a Since the primary thread of every core is performing a wrmsr
	for the update, once the wrmsr has started, it can't be
	interrupted. Hence its not required to NMI the primary thread of
	the core. Only the secondary thread needs to be parked in NMI
	before the update begins.
	Suggested by From Andy Cooper
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.

   [ Lutomirsky thinks this is weak, and what happens from taking the
   interrupt and the path to the registered callback handler might be
   exposed.]

   Seems like 1.a is the best candidate.

The algorithm is something like this:

After stop_machine() all threads are executing __reload_late()

nmi_callback()
{
	if (!in_ucode_update)
		return NMI_DONE;
	if (cpu not in sibling_mask)
		return NMI_DONE;
	update sibling reached NMI for primary to continue

	while (cpu in sibling_mask)
		wait;
	return NMI_HANDLED;
}

__reload_late()
{

	entry_rendezvous(&late_cpus_in);
	set_mcip()
	if (this_cpu is first_cpu in the core)
		wait for siblings to drop in NMI
		apply_microcode()
	else
		wait_for_siblings;

wait_for_siblings:

	exit_rendezvous(&late_cpus_out);
	clear_mcip
}

reload_late()
{
	register_nmi_handler()
	prepare_mask of all sibling cpus()
	update state = ucode in progress;
	send NMI to all threads in sibling_mask
	unregister_nmi_handler();
}

Signed-off-by: Ashok Raj <ashok.raj@xxxxxxxxx>
---
 arch/x86/kernel/cpu/microcode/core.c | 191 ++++++++++++++++++++++++++-
 1 file changed, 184 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index d24e1c754c27..26ceb4e37a53 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -39,7 +39,9 @@
 #include <asm/processor.h>
 #include <asm/cmdline.h>
 #include <asm/setup.h>
+#include <asm/apic.h>
 #include <asm/mce.h>
+#include <asm/nmi.h>
 
 #define DRIVER_VERSION	"2.2"
 
@@ -411,6 +413,12 @@ static int check_online_cpus(void)
 
 static atomic_t late_cpus_in;
 static atomic_t late_cpus_out;
+static atomic_t nmi_cpus;	// number of CPUs that enter NMI
+static atomic_t nmi_timeouts;   // number of siblings that timeout
+static atomic_t nmi_siblings;   // Nmber of siblings that enter NMI
+static atomic_t in_ucode_update;// Are we in microcode update?
+
+static struct cpumask all_sibling_mask;
 
 static int __wait_for_cpus(atomic_t *t, long long timeout)
 {
@@ -433,6 +441,74 @@ static int __wait_for_cpus(atomic_t *t, long long timeout)
 	return 0;
 }
 
+struct core_rendez {
+	int num_core_cpus;
+	atomic_t callin;
+};
+
+static DEFINE_PER_CPU(struct core_rendez, core_sync);
+
+static int ucode_nmi_cb(unsigned int val, struct pt_regs *regs)
+{
+	int first_cpu, cpu = smp_processor_id();
+	int timeout = 100 * NSEC_PER_USEC;
+	struct core_rendez *rendez;
+
+	atomic_inc(&nmi_cpus);
+	if (!atomic_read(&in_ucode_update))
+		return NMI_DONE;
+
+	if (!cpumask_test_cpu(cpu, &all_sibling_mask))
+		return NMI_DONE;
+
+	atomic_inc(&nmi_siblings);
+	pr_debug("CPU %d made into NMI handler\n", cpu);
+	/*
+	 * primary thread waits for this
+	 * before performing update
+	 */
+	first_cpu = cpumask_first(topology_sibling_cpumask(cpu));
+	rendez = &per_cpu(core_sync, first_cpu);
+
+	atomic_inc(&rendez->callin);
+	while (timeout < NSEC_PER_USEC) {
+		if (timeout < NSEC_PER_USEC) {
+			atomic_inc(&nmi_timeouts);
+			pr_debug("CPU %d sibling timedout\n",cpu);
+			break;
+		}
+		ndelay(NSEC_PER_USEC);
+		timeout -= NSEC_PER_USEC;
+		touch_nmi_watchdog();
+		/*
+		 * Once primary clears it from all_sibling_mask, we are
+		 * released from the NMI loop
+		 */
+		if (!cpumask_test_cpu(cpu, &all_sibling_mask)) {
+			pr_debug("CPU %d breaking from NMI\n",cpu);
+			break;
+		}
+	}
+	return NMI_HANDLED;
+}
+
+/*
+ * Primary thread clears the cpumask to release the siblings from the NMI
+ * jail
+ */
+
+static void clear_nmi_cpus(void)
+{
+	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, &all_sibling_mask);
+	}
+}
+
 /*
  * Returns:
  * < 0 - on error
@@ -440,14 +516,14 @@ 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;
 
 	/*
 	 * Wait for all CPUs to arrive. A load will not be attempted unless all
 	 * CPUs show up.
-	 * */
+	 */
 	if (__wait_for_cpus(&late_cpus_in, NSEC_PER_SEC))
 		return -1;
 
@@ -459,6 +535,7 @@ 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.
@@ -466,13 +543,47 @@ 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) {
+		struct core_rendez *pcpu_core = &per_cpu(core_sync, cpu);
+		int num_sibs = pcpu_core->num_core_cpus - 1;
+		int timeout = 100 * NSEC_PER_USEC;
+
+		/*
+		 * Wait for all siblings to enter
+		 * NMI before performing the update
+		 */
+		while (atomic_read(&pcpu_core->callin) < num_sibs) {
+			if (timeout < NSEC_PER_USEC) {
+				timeout -= NSEC_PER_USEC;
+				ndelay(NSEC_PER_USEC);
+				touch_nmi_watchdog();
+			} else {
+				/*
+				 * Some unknown reason the siblings didn't
+				 * check in with primary. We just signal
+				 * failure, but better to update since
+				 * other cores might be successful and we
+				 * don't want to leave this alone
+				 */
+				pr_err("CPU %d core lead timeout waiting for"
+				       " siblings\n", cpu);
+				ret = -1;
+				break;
+			}
+		}
+		pr_debug("Primary CPU %d proceeding with update\n", cpu);
 		apply_microcode_local(&err);
-	else
+		clear_nmi_cpus();
+	} else
 		goto wait_for_siblings;
 
-	if (err >= UCODE_NFOUND) {
+	if (ret || err >= UCODE_NFOUND) {
 		if (err == UCODE_ERROR)
 			pr_warn("Error reloading microcode on CPU %d\n", cpu);
 
@@ -496,26 +607,92 @@ static int __reload_late(void *info)
 	return ret;
 }
 
+static void set_nmi_cpus(int cpu)
+{
+	int first_cpu, wait_cpu;
+	struct core_rendez *pcpu_core = &per_cpu(core_sync, cpu);
+
+	first_cpu = cpumask_first(topology_sibling_cpumask(cpu));
+	for_each_cpu(wait_cpu, topology_sibling_cpumask(cpu)) {
+		if (wait_cpu == first_cpu) {
+			pcpu_core->num_core_cpus =
+					cpumask_weight(topology_sibling_cpumask(wait_cpu));
+			continue;
+		}
+		cpumask_set_cpu(wait_cpu, &all_sibling_mask);
+	}
+}
+
+static void prepare_siblings(void)
+{
+	int cpu;
+
+	for_each_cpu(cpu, cpu_online_mask) {
+		set_nmi_cpus(cpu);
+	}
+}
+
 /*
  * Reload microcode late on all CPUs. Wait for a sec until they
  * all gather together.
  */
 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");
 
+	/*
+	 * Used for late_load entry and exit rendezvous
+	 */
 	atomic_set(&late_cpus_in,  0);
 	atomic_set(&late_cpus_out, 0);
 
+	/*
+	 * in_ucode_update: Global state while in ucode update
+	 * nmi_cpus: Count of CPUs entering NMI while ucode in progress
+	 * nmi_siblings: Count of siblings that enter NMI
+	 * nmi_timeouts: Count of siblings that fail to see mask clear
+	 */
+	atomic_set(&in_ucode_update,0);
+	atomic_set(&nmi_cpus, 0);
+	atomic_set(&nmi_timeouts, 0);
+	atomic_set(&nmi_siblings, 0);
+
+	cpumask_clear(&all_sibling_mask);
+
+	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;
+	}
+
+	/*
+	 * Prepare everything for siblings threads to drop into NMI while
+	 * the update is in progress.
+	 */
+	prepare_siblings();
+	atomic_set(&in_ucode_update, 1);
+	apic->send_IPI_mask(&all_sibling_mask, NMI_VECTOR);
+	pr_debug("Sent NMI broadcast to all sibling cpus\n");
 	ret = stop_machine_cpuslocked(__reload_late, NULL, cpu_online_mask);
 	if (ret == 0)
 		microcode_check();
 
-	pr_info("Reload completed, microcode revision: 0x%x\n", boot_cpu_data.microcode);
+	unregister_nmi_handler(NMI_LOCAL, "ucode_nmi");
+
+	pr_debug("Total CPUs that entered NMI     ... %d\n",
+			atomic_read(&nmi_cpus));
+	pr_debug("Total siblings that entered NMI ... %d\n",
+		  	atomic_read(&nmi_siblings));
+	pr_debug("Total siblings timedout         ... %d\n",
+			atomic_read(&nmi_timeouts));
+	pr_info("Reload completed, microcode revision: 0x%x\n",
+	        boot_cpu_data.microcode);
 
+done:
 	return ret;
 }
 
-- 
2.32.0