ARM: 7807/1: kexec: validate CPU hotplug support

Architectures should fully validate whether kexec is possible as part of
machine_kexec_prepare(), so that user-space's kexec_load() operation can
report any problems. Performing validation in machine_kexec() itself is
too late, since it is not allowed to return.

Prior to this patch, ARM's machine_kexec() was testing after-the-fact
whether machine_kexec_prepare() was able to disable all but one CPU.
Instead, modify machine_kexec_prepare() to validate all conditions
necessary for machine_kexec_prepare()'s to succeed. BUG if the validation
succeeded, yet disabling the CPUs didn't actually work.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

3 files changed, 29 insertions, 4 deletions

diff --git a/arch/arm/include/asm/smp_plat.h b/arch/arm/include/asm/smp_plat.h
index 6462a721ebd4..a252c0bfacf5 100644
--- a/arch/arm/include/asm/smp_plat.h
+++ b/arch/arm/include/asm/smp_plat.h
@@ -88,4 +88,7 @@ static inline u32 mpidr_hash_size(void)
 {
         return 1 << mpidr_hash.bits;
 }
+
+extern int platform_can_cpu_hotplug(void);
+
 #endif
diff --git a/arch/arm/kernel/machine_kexec.c b/arch/arm/kernel/machine_kexec.c
index 4fb074c446bf..d7c82df69243 100644
--- a/arch/arm/kernel/machine_kexec.c
+++ b/arch/arm/kernel/machine_kexec.c
@@ -15,6 +15,7 @@
 #include <asm/mmu_context.h>
 #include <asm/cacheflush.h>
 #include <asm/mach-types.h>
+#include <asm/smp_plat.h>
 #include <asm/system_misc.h>
 
 extern const unsigned char relocate_new_kernel[];
@@ -39,6 +40,14 @@ int machine_kexec_prepare(struct kimage *image)
         int i, err;
 
         /*
+         * Validate that if the current HW supports SMP, then the SW supports
+         * and implements CPU hotplug for the current HW. If not, we won't be
+         * able to kexec reliably, so fail the prepare operation.
+         */
+        if (num_possible_cpus() > 1 && !platform_can_cpu_hotplug())
+                return -EINVAL;
+
+        /*
          * No segment at default ATAGs address. try to locate
          * a dtb using magic.
          */
@@ -134,10 +143,13 @@ void machine_kexec(struct kimage *image)
         unsigned long reboot_code_buffer_phys;
         void *reboot_code_buffer;
 
-        if (num_online_cpus() > 1) {
-                pr_err("kexec: error: multiple CPUs still online\n");
-                return;
-        }
+        /*
+         * This can only happen if machine_shutdown() failed to disable some
+         * CPU, and that can only happen if the checks in
+         * machine_kexec_prepare() were not correct. If this fails, we can't
+         * reliably kexec anyway, so BUG_ON is appropriate.
+         */
+        BUG_ON(num_online_cpus() > 1);
 
         page_list = image->head & PAGE_MASK;
 
diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index c2b4f8f0be9a..2dc19349eb19 100644
--- a/arch/arm/kernel/smp.c
+++ b/arch/arm/kernel/smp.c
@@ -145,6 +145,16 @@ int boot_secondary(unsigned int cpu, struct task_struct *idle)
         return -ENOSYS;
 }
 
+int platform_can_cpu_hotplug(void)
+{
+#ifdef CONFIG_HOTPLUG_CPU
+        if (smp_ops.cpu_kill)
+                return 1;
+#endif
+
+        return 0;
+}
+
 #ifdef CONFIG_HOTPLUG_CPU
 static void percpu_timer_stop(void);