ARM: Check if a CPU has gone offline

PSCIv0.2 adds a new function called AFFINITY_INFO, which
can be used to query if a specified CPU has actually gone
offline. Calling this function via cpu_kill ensures that
a CPU has quiesced after a call to cpu_die. This helps
prevent the CPU from doing arbitrary bad things when data
or instructions are clobbered (as happens with kexec)
in the window between a CPU announcing that it is dead
and said CPU leaving the kernel.

Signed-off-by: Ashwin Chaugule <ashwin.chaugule@linaro.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>

3 files changed, 55 insertions, 0 deletions

diff --git a/arch/arm64/include/asm/cpu_ops.h b/arch/arm64/include/asm/cpu_ops.h
index 152413076503..d7b4b38a8e86 100644
--- a/arch/arm64/include/asm/cpu_ops.h
+++ b/arch/arm64/include/asm/cpu_ops.h
@@ -39,6 +39,7 @@ struct device_node;
  *              from the cpu to be killed.
  * @cpu_die:    Makes a cpu leave the kernel. Must not fail. Called from the
  *              cpu being killed.
+ * @cpu_kill:  Ensures a cpu has left the kernel. Called from another cpu.
  * @cpu_suspend: Suspends a cpu and saves the required context. May fail owing
  *               to wrong parameters or error conditions. Called from the
  *               CPU being suspended. Must be called with IRQs disabled.
@@ -52,6 +53,7 @@ struct cpu_operations {
 #ifdef CONFIG_HOTPLUG_CPU
         int             (*cpu_disable)(unsigned int cpu);
         void            (*cpu_die)(unsigned int cpu);
+        int             (*cpu_kill)(unsigned int cpu);
 #endif
 #ifdef CONFIG_ARM64_CPU_SUSPEND
         int             (*cpu_suspend)(unsigned long);
diff --git a/arch/arm64/kernel/psci.c b/arch/arm64/kernel/psci.c
index 90df6e641227..9e9798f91172 100644
--- a/arch/arm64/kernel/psci.c
+++ b/arch/arm64/kernel/psci.c
@@ -20,6 +20,7 @@
 #include <linux/smp.h>
 #include <linux/reboot.h>
 #include <linux/pm.h>
+#include <linux/delay.h>
 #include <uapi/linux/psci.h>
 
 #include <asm/compiler.h>
@@ -403,6 +404,35 @@ static void cpu_psci_cpu_die(unsigned int cpu)
 
         pr_crit("unable to power off CPU%u (%d)\n", cpu, ret);
 }
+
+static int cpu_psci_cpu_kill(unsigned int cpu)
+{
+        int err, i;
+
+        if (!psci_ops.affinity_info)
+                return 1;
+        /*
+         * cpu_kill could race with cpu_die and we can
+         * potentially end up declaring this cpu undead
+         * while it is dying. So, try again a few times.
+         */
+
+        for (i = 0; i < 10; i++) {
+                err = psci_ops.affinity_info(cpu_logical_map(cpu), 0);
+                if (err == PSCI_0_2_AFFINITY_LEVEL_OFF) {
+                        pr_info("CPU%d killed.\n", cpu);
+                        return 1;
+                }
+
+                msleep(10);
+                pr_info("Retrying again to check for CPU kill\n");
+        }
+
+        pr_warn("CPU%d may not have shut down cleanly (AFFINITY_INFO reports %d)\n",
+                        cpu, err);
+        /* Make op_cpu_kill() fail. */
+        return 0;
+}
 #endif
 
 const struct cpu_operations cpu_psci_ops = {
@@ -413,6 +443,7 @@ const struct cpu_operations cpu_psci_ops = {
 #ifdef CONFIG_HOTPLUG_CPU
         .cpu_disable    = cpu_psci_cpu_disable,
         .cpu_die        = cpu_psci_cpu_die,
+        .cpu_kill       = cpu_psci_cpu_kill,
 #endif
 };
 
diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index f0a141dd5655..c3cb160edc69 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -228,6 +228,19 @@ int __cpu_disable(void)
         return 0;
 }
 
+static int op_cpu_kill(unsigned int cpu)
+{
+        /*
+         * If we have no means of synchronising with the dying CPU, then assume
+         * that it is really dead. We can only wait for an arbitrary length of
+         * time and hope that it's dead, so let's skip the wait and just hope.
+         */
+        if (!cpu_ops[cpu]->cpu_kill)
+                return 1;
+
+        return cpu_ops[cpu]->cpu_kill(cpu);
+}
+
 static DECLARE_COMPLETION(cpu_died);
 
 /*
@@ -241,6 +254,15 @@ void __cpu_die(unsigned int cpu)
                 return;
         }
         pr_notice("CPU%u: shutdown\n", cpu);
+
+        /*
+         * Now that the dying CPU is beyond the point of no return w.r.t.
+         * in-kernel synchronisation, try to get the firwmare to help us to
+         * verify that it has really left the kernel before we consider
+         * clobbering anything it might still be using.
+         */
+        if (!op_cpu_kill(cpu))
+                pr_warn("CPU%d may not have shut down cleanly\n", cpu);
 }
 
 /*