x86/microcode: Rip out the subsys interface gunk

This is a left-over from the old days when CPU hotplug wasn't as robust
as it is now. Currently, microcode gets loaded early on the CPU init
path and there's no need to attempt to load it again, which that subsys
interface callback is doing.

The only other thing that the subsys interface init path was doing is
adding the

  /sys/devices/system/cpu/cpu*/microcode/

hierarchy.

So add a function which gets called on each CPU after all the necessary
driver setup has happened. Use schedule_on_each_cpu() which can block
because the sysfs creating code does kmem_cache_zalloc() which can block
too and the initial version of this where it did that setup in an IPI
handler of on_each_cpu() can cause a deadlock of the sort:

  lock(fs_reclaim);
  <Interrupt>
    lock(fs_reclaim);

as the IPI handler runs in IRQ context.
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Ashok Raj <ashok.raj@intel.com>
Link: https://lore.kernel.org/r/20221028142638.28498-2-bp@alien8.de

arch/x86/kernel/cpu/microcode/core.c

@@ -601,8 +601,8 @@ static enum ucode_state microcode_resume_cpu(int cpu)
 
 static enum ucode_state microcode_init_cpu(int cpu, bool refresh_fw)
 {
-	enum ucode_state ustate;
 	struct ucode_cpu_info *uci = ucode_cpu_info + cpu;
+	enum ucode_state ustate;
 
 	if (uci->valid)
 		return UCODE_OK;
@@ -636,44 +636,6 @@ static enum ucode_state microcode_update_cpu(int cpu)
 	return microcode_init_cpu(cpu, false);
 }
 
-static int mc_device_add(struct device *dev, struct subsys_interface *sif)
-{
-	int err, cpu = dev->id;
-
-	if (!cpu_online(cpu))
-		return 0;
-
-	pr_debug("CPU%d added\n", cpu);
-
-	err = sysfs_create_group(&dev->kobj, &mc_attr_group);
-	if (err)
-		return err;
-
-	if (microcode_init_cpu(cpu, true) == UCODE_ERROR)
-		return -EINVAL;
-
-	return err;
-}
-
-static void mc_device_remove(struct device *dev, struct subsys_interface *sif)
-{
-	int cpu = dev->id;
-
-	if (!cpu_online(cpu))
-		return;
-
-	pr_debug("CPU%d removed\n", cpu);
-	microcode_fini_cpu(cpu);
-	sysfs_remove_group(&dev->kobj, &mc_attr_group);
-}
-
-static struct subsys_interface mc_cpu_interface = {
-	.name			= "microcode",
-	.subsys			= &cpu_subsys,
-	.add_dev		= mc_device_add,
-	.remove_dev		= mc_device_remove,
-};
-
 /**
  * microcode_bsp_resume - Update boot CPU microcode during resume.
  */
@@ -713,6 +675,9 @@ static int mc_cpu_down_prep(unsigned int cpu)
 	struct device *dev;
 
 	dev = get_cpu_device(cpu);
+
+	microcode_fini_cpu(cpu);
+
 	/* Suspend is in progress, only remove the interface */
 	sysfs_remove_group(&dev->kobj, &mc_attr_group);
 	pr_debug("CPU%d removed\n", cpu);
@@ -720,6 +685,18 @@ static int mc_cpu_down_prep(unsigned int cpu)
 	return 0;
 }
 
+static void setup_online_cpu(struct work_struct *work)
+{
+	int cpu = smp_processor_id();
+	struct ucode_cpu_info *uci = ucode_cpu_info + cpu;
+
+	memset(uci, 0, sizeof(*uci));
+
+	microcode_ops->collect_cpu_info(cpu, &uci->cpu_sig);
+
+	mc_cpu_online(cpu);
+}
+
 static struct attribute *cpu_root_microcode_attrs[] = {
 #ifdef CONFIG_MICROCODE_LATE_LOADING
 	&dev_attr_reload.attr,
@@ -755,23 +732,17 @@ static int __init microcode_init(void)
 	if (IS_ERR(microcode_pdev))
 		return PTR_ERR(microcode_pdev);
 
-	cpus_read_lock();
-	mutex_lock(&microcode_mutex);
-	error = subsys_interface_register(&mc_cpu_interface);
-	mutex_unlock(&microcode_mutex);
-	cpus_read_unlock();
-
-	if (error)
-		goto out_pdev;
-
 	error = sysfs_create_group(&cpu_subsys.dev_root->kobj,
 				   &cpu_root_microcode_group);
 
 	if (error) {
 		pr_err("Error creating microcode group!\n");
-		goto out_driver;
+		goto out_pdev;
 	}
 
+	/* Do per-CPU setup */
+	schedule_on_each_cpu(setup_online_cpu);
+
 	register_syscore_ops(&mc_syscore_ops);
 	cpuhp_setup_state_nocalls(CPUHP_AP_MICROCODE_LOADER, "x86/microcode:starting",
 				  mc_cpu_starting, NULL);
@@ -782,15 +753,6 @@ static int __init microcode_init(void)
 
 	return 0;
 
- out_driver:
-	cpus_read_lock();
-	mutex_lock(&microcode_mutex);
-
-	subsys_interface_unregister(&mc_cpu_interface);
-
-	mutex_unlock(&microcode_mutex);
-	cpus_read_unlock();
-
  out_pdev:
 	platform_device_unregister(microcode_pdev);
 	return error;