x86: make get_cpu_vendor() accessible from Xen code

In order to be able to differentiate between AMD and Intel based
systems for very early hypercalls without having to rely on the Xen
hypercall page, make get_cpu_vendor() non-static.

Refactor early_cpu_init() for the same reason by splitting out the
loop initializing cpu_devs() into an externally callable function.

This is part of XSA-466 / CVE-2024-53241.

Reported-by: Andrew Cooper <[e-mail]>
Signed-off-by: Juergen Gross <[e-mail]>

arch/x86/include/asm/processor.h

···
 	return BIT_ULL(boot_cpu_data.x86_cache_bits - 1 - PAGE_SHIFT);
 }
 
+void init_cpu_devs(void);
+void get_cpu_vendor(struct cpuinfo_x86 *c);
 extern void early_cpu_init(void);
 extern void identify_secondary_cpu(struct cpuinfo_x86 *);
 extern void print_cpu_info(struct cpuinfo_x86 *);

arch/x86/kernel/cpu/common.c

···
 		tlb_lld_4m[ENTRIES], tlb_lld_1g[ENTRIES]);
 }
 
-static void get_cpu_vendor(struct cpuinfo_x86 *c)
+void get_cpu_vendor(struct cpuinfo_x86 *c)
 {
 	char *v = c->x86_vendor_id;
 	int i;
···
 	detect_nopl();
 }
 
-void __init early_cpu_init(void)
+void __init init_cpu_devs(void)
 {
 	const struct cpu_dev *const *cdev;
 	int count = 0;
-
-#ifdef CONFIG_PROCESSOR_SELECT
-	pr_info("KERNEL supported cpus:\n");
-#endif
 
 	for (cdev = __x86_cpu_dev_start; cdev < __x86_cpu_dev_end; cdev++) {
 		const struct cpu_dev *cpudev = *cdev;
···
 			break;
 		cpu_devs[count] = cpudev;
 		count++;
+	}
+}
+
+void __init early_cpu_init(void)
+{
+#ifdef CONFIG_PROCESSOR_SELECT
+	unsigned int i, j;
+
+	pr_info("KERNEL supported cpus:\n");
+#endif
+
+	init_cpu_devs();
 
 #ifdef CONFIG_PROCESSOR_SELECT
-		{
-			unsigned int j;
-
-			for (j = 0; j < 2; j++) {
-				if (!cpudev->c_ident[j])
-					continue;
-				pr_info("  %s %s\n", cpudev->c_vendor,
-					cpudev->c_ident[j]);
-			}
+	for (i = 0; i < X86_VENDOR_NUM && cpu_devs[i]; i++) {
+		for (j = 0; j < 2; j++) {
+			if (!cpu_devs[i]->c_ident[j])
+				continue;
+			pr_info("  %s %s\n", cpu_devs[i]->c_vendor,
+				cpu_devs[i]->c_ident[j]);
 		}
-#endif
 	}
+#endif
+
 	early_identify_cpu(&boot_cpu_data);
 }