x86/MSR: make msr_class a static const structure

Now that the driver core allows for struct class to be in read-only
memory, move the msr_class structure to be declared at build time
placing it into read-only memory, instead of having to be dynamically
allocated at boot time.

Cc: "H. Peter Anvin" <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: [e-mail]
Suggested-by: Greg Kroah-Hartman <[e-mail]>
Signed-off-by: Ivan Orlov <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

arch/x86/kernel/msr.c

···
 #include <asm/cpufeature.h>
 #include <asm/msr.h>
 
-static struct class *msr_class;
 static enum cpuhp_state cpuhp_msr_state;
 
 enum allow_write_msrs {
···
 	.compat_ioctl = msr_ioctl,
 };
 
+static char *msr_devnode(const struct device *dev, umode_t *mode)
+{
+	return kasprintf(GFP_KERNEL, "cpu/%u/msr", MINOR(dev->devt));
+}
+
+static const struct class msr_class = {
+	.name		= "msr",
+	.devnode	= msr_devnode,
+};
+
 static int msr_device_create(unsigned int cpu)
 {
 	struct device *dev;
 
-	dev = device_create(msr_class, NULL, MKDEV(MSR_MAJOR, cpu), NULL,
+	dev = device_create(&msr_class, NULL, MKDEV(MSR_MAJOR, cpu), NULL,
 			    "msr%d", cpu);
 	return PTR_ERR_OR_ZERO(dev);
 }
 
 static int msr_device_destroy(unsigned int cpu)
 {
-	device_destroy(msr_class, MKDEV(MSR_MAJOR, cpu));
+	device_destroy(&msr_class, MKDEV(MSR_MAJOR, cpu));
 	return 0;
-}
-
-static char *msr_devnode(const struct device *dev, umode_t *mode)
-{
-	return kasprintf(GFP_KERNEL, "cpu/%u/msr", MINOR(dev->devt));
 }
 
 static int __init msr_init(void)
···
 		pr_err("unable to get major %d for msr\n", MSR_MAJOR);
 		return -EBUSY;
 	}
-	msr_class = class_create("msr");
-	if (IS_ERR(msr_class)) {
-		err = PTR_ERR(msr_class);
+	err = class_register(&msr_class);
+	if (err)
 		goto out_chrdev;
-	}
-	msr_class->devnode = msr_devnode;
 
 	err  = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "x86/msr:online",
 				 msr_device_create, msr_device_destroy);
···
 	return 0;
 
 out_class:
-	class_destroy(msr_class);
+	class_unregister(&msr_class);
 out_chrdev:
 	__unregister_chrdev(MSR_MAJOR, 0, NR_CPUS, "cpu/msr");
 	return err;
···
 static void __exit msr_exit(void)
 {
 	cpuhp_remove_state(cpuhp_msr_state);
-	class_destroy(msr_class);
+	class_unregister(&msr_class);
 	__unregister_chrdev(MSR_MAJOR, 0, NR_CPUS, "cpu/msr");
 }
 module_exit(msr_exit)