x86/microcode: Fix CPU synchronization routine

[Linux-MTD Mailing List]

Gitweb:     http://git.infradead.org/?p=mtd-2.6.git;a=commit;h=bb8c13d61a629276a162c1d2b1a20a815cbcfbb7
Commit:     bb8c13d61a629276a162c1d2b1a20a815cbcfbb7
Parent:     2613f36ed965d0e5a595a1d931fd3b480e82d6fd
Author:     Borislav Petkov <bp at suse.de>
AuthorDate: Wed Mar 14 19:36:15 2018 +0100
Committer:  Thomas Gleixner <tglx at linutronix.de>
CommitDate: Fri Mar 16 20:55:51 2018 +0100

    x86/microcode: Fix CPU synchronization routine
    
    Emanuel reported an issue with a hang during microcode update because my
    dumb idea to use one atomic synchronization variable for both rendezvous
    - before and after update - was simply bollocks:
    
      microcode: microcode_reload_late: late_cpus: 4
      microcode: __reload_late: cpu 2 entered
      microcode: __reload_late: cpu 1 entered
      microcode: __reload_late: cpu 3 entered
      microcode: __reload_late: cpu 0 entered
      microcode: __reload_late: cpu 1 left
      microcode: Timeout while waiting for CPUs rendezvous, remaining: 1
    
    CPU1 above would finish, leave and the others will still spin waiting for
    it to join.
    
    So do two synchronization atomics instead, which makes the code a lot more
    straightforward.
    
    Also, since the update is serialized and it also takes quite some time per
    microcode engine, increase the exit timeout by the number of CPUs on the
    system.
    
    That's ok because the moment all CPUs are done, that timeout will be cut
    short.
    
    Furthermore, panic when some of the CPUs timeout when returning from a
    microcode update: we can't allow a system with not all cores updated.
    
    Also, as an optimization, do not do the exit sync if microcode wasn't
    updated.
    
    Reported-by: Emanuel Czirai <xftroxgpx at protonmail.com>
    Signed-off-by: Borislav Petkov <bp at suse.de>
    Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
    Tested-by: Emanuel Czirai <xftroxgpx at protonmail.com>
    Tested-by: Ashok Raj <ashok.raj at intel.com>
    Tested-by: Tom Lendacky <thomas.lendacky at amd.com>
    Link: https://lkml.kernel.org/r/20180314183615.17629-2-bp@alien8.de
---
 arch/x86/kernel/cpu/microcode/core.c | 68 ++++++++++++++++++++++--------------
 1 file changed, 41 insertions(+), 27 deletions(-)

diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index 9f0fe5bb450d..10c4fc2c91f8 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -517,7 +517,29 @@ static int check_online_cpus(void)
 	return -EINVAL;
 }
 
-static atomic_t late_cpus;
+static atomic_t late_cpus_in;
+static atomic_t late_cpus_out;
+
+static int __wait_for_cpus(atomic_t *t, long long timeout)
+{
+	int all_cpus = num_online_cpus();
+
+	atomic_inc(t);
+
+	while (atomic_read(t) < all_cpus) {
+		if (timeout < SPINUNIT) {
+			pr_err("Timeout while waiting for CPUs rendezvous, remaining: %d\n",
+				all_cpus - atomic_read(t));
+			return 1;
+		}
+
+		ndelay(SPINUNIT);
+		timeout -= SPINUNIT;
+
+		touch_nmi_watchdog();
+	}
+	return 0;
+}
 
 /*
  * Returns:
@@ -527,30 +549,16 @@ static atomic_t late_cpus;
  */
 static int __reload_late(void *info)
 {
-	unsigned int timeout = NSEC_PER_SEC;
-	int all_cpus = num_online_cpus();
 	int cpu = smp_processor_id();
 	enum ucode_state err;
 	int ret = 0;
 
-	atomic_dec(&late_cpus);
-
 	/*
 	 * Wait for all CPUs to arrive. A load will not be attempted unless all
 	 * CPUs show up.
 	 * */
-	while (atomic_read(&late_cpus)) {
-		if (timeout < SPINUNIT) {
-			pr_err("Timeout while waiting for CPUs rendezvous, remaining: %d\n",
-				atomic_read(&late_cpus));
-			return -1;
-		}
-
-		ndelay(SPINUNIT);
-		timeout -= SPINUNIT;
-
-		touch_nmi_watchdog();
-	}
+	if (__wait_for_cpus(&late_cpus_in, NSEC_PER_SEC))
+		return -1;
 
 	spin_lock(&update_lock);
 	apply_microcode_local(&err);
@@ -558,15 +566,22 @@ static int __reload_late(void *info)
 
 	if (err > UCODE_NFOUND) {
 		pr_warn("Error reloading microcode on CPU %d\n", cpu);
-		ret = -1;
-	} else if (err == UCODE_UPDATED) {
+		return -1;
+	/* siblings return UCODE_OK because their engine got updated already */
+	} else if (err == UCODE_UPDATED || err == UCODE_OK) {
 		ret = 1;
+	} else {
+		return ret;
 	}
 
-	atomic_inc(&late_cpus);
-
-	while (atomic_read(&late_cpus) != all_cpus)
-		cpu_relax();
+	/*
+	 * Increase the wait timeout to a safe value here since we're
+	 * serializing the microcode update and that could take a while on a
+	 * large number of CPUs. And that is fine as the *actual* timeout will
+	 * be determined by the last CPU finished updating and thus cut short.
+	 */
+	if (__wait_for_cpus(&late_cpus_out, NSEC_PER_SEC * num_online_cpus()))
+		panic("Timeout during microcode update!\n");
 
 	return ret;
 }
@@ -579,12 +594,11 @@ static int microcode_reload_late(void)
 {
 	int ret;
 
-	atomic_set(&late_cpus, num_online_cpus());
+	atomic_set(&late_cpus_in,  0);
+	atomic_set(&late_cpus_out, 0);
 
 	ret = stop_machine_cpuslocked(__reload_late, NULL, cpu_online_mask);
-	if (ret < 0)
-		return ret;
-	else if (ret > 0)
+	if (ret > 0)
 		microcode_check();
 
 	return ret;