Fix a boot regression on very old x86 CPUs without CPUID support.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
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Merge tag 'x86-urgent-2025-05-11' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 fix from Ingo Molnar:
 "Fix a boot regression on very old x86 CPUs without CPUID support"

* tag 'x86-urgent-2025-05-11' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/microcode: Consolidate the loader enablement checking

arch/x86/include/asm/microcode.h

@@ -17,10 +17,12 @@ struct ucode_cpu_info {
 void load_ucode_bsp(void);
 void load_ucode_ap(void);
 void microcode_bsp_resume(void);
+bool __init microcode_loader_disabled(void);
 #else
 static inline void load_ucode_bsp(void)	{ }
 static inline void load_ucode_ap(void) { }
 static inline void microcode_bsp_resume(void) { }
+static inline bool __init microcode_loader_disabled(void) { return false; }
 #endif
 
 extern unsigned long initrd_start_early;

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

@@ -1098,15 +1098,17 @@ static enum ucode_state load_microcode_amd(u8 family, const u8 *data, size_t siz
 
 static int __init save_microcode_in_initrd(void)
 {
-	unsigned int cpuid_1_eax = native_cpuid_eax(1);
 	struct cpuinfo_x86 *c = &boot_cpu_data;
 	struct cont_desc desc = { 0 };
+	unsigned int cpuid_1_eax;
 	enum ucode_state ret;
 	struct cpio_data cp;
 
-	if (dis_ucode_ldr || c->x86_vendor != X86_VENDOR_AMD || c->x86 < 0x10)
+	if (microcode_loader_disabled() || c->x86_vendor != X86_VENDOR_AMD || c->x86 < 0x10)
 		return 0;
 
+	cpuid_1_eax = native_cpuid_eax(1);
+
 	if (!find_blobs_in_containers(&cp))
 		return -EINVAL;
 

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

@@ -42,7 +42,7 @@
 #include "internal.h"
 
 static struct microcode_ops *microcode_ops;
-bool dis_ucode_ldr = true;
+static bool dis_ucode_ldr = false;
 
 bool force_minrev = IS_ENABLED(CONFIG_MICROCODE_LATE_FORCE_MINREV);
 module_param(force_minrev, bool, S_IRUSR | S_IWUSR);
@@ -84,6 +84,9 @@ static bool amd_check_current_patch_level(void)
 	u32 lvl, dummy, i;
 	u32 *levels;
 
+	if (x86_cpuid_vendor() != X86_VENDOR_AMD)
+		return false;
+
 	native_rdmsr(MSR_AMD64_PATCH_LEVEL, lvl, dummy);
 
 	levels = final_levels;
@@ -95,27 +98,29 @@ static bool amd_check_current_patch_level(void)
 	return false;
 }
 
-static bool __init check_loader_disabled_bsp(void)
+bool __init microcode_loader_disabled(void)
 {
-	static const char *__dis_opt_str = "dis_ucode_ldr";
+	if (dis_ucode_ldr)
-	const char *cmdline = boot_command_line;
+		return true;
-	const char *option  = __dis_opt_str;
 
 	/*
-	 * CPUID(1).ECX[31]: reserved for hypervisor use. This is still not
+	 * Disable when:
-	 * completely accurate as xen pv guests don't see that CPUID bit set but
+	 *
-	 * that's good enough as they don't land on the BSP path anyway.
+	 * 1) The CPU does not support CPUID.
+	 *
+	 * 2) Bit 31 in CPUID[1]:ECX is clear
+	 *    The bit is reserved for hypervisor use. This is still not
+	 *    completely accurate as XEN PV guests don't see that CPUID bit
+	 *    set, but that's good enough as they don't land on the BSP
+	 *    path anyway.
+	 *
+	 * 3) Certain AMD patch levels are not allowed to be
+	 *    overwritten.
 	 */
-	if (native_cpuid_ecx(1) & BIT(31))
+	if (!have_cpuid_p() ||
-		return true;
+	    native_cpuid_ecx(1) & BIT(31) ||
-
+	    amd_check_current_patch_level())
-	if (x86_cpuid_vendor() == X86_VENDOR_AMD) {
+		dis_ucode_ldr = true;
-		if (amd_check_current_patch_level())
-			return true;
-	}
-
-	if (cmdline_find_option_bool(cmdline, option) <= 0)
-		dis_ucode_ldr = false;
 
 	return dis_ucode_ldr;
 }
@@ -125,7 +130,10 @@ void __init load_ucode_bsp(void)
 	unsigned int cpuid_1_eax;
 	bool intel = true;
 
-	if (!have_cpuid_p())
+	if (cmdline_find_option_bool(boot_command_line, "dis_ucode_ldr") > 0)
+		dis_ucode_ldr = true;
+
+	if (microcode_loader_disabled())
 		return;
 
 	cpuid_1_eax = native_cpuid_eax(1);
@@ -146,9 +154,6 @@ void __init load_ucode_bsp(void)
 		return;
 	}
 
-	if (check_loader_disabled_bsp())
-		return;
-
 	if (intel)
 		load_ucode_intel_bsp(&early_data);
 	else
@@ -159,6 +164,11 @@ void load_ucode_ap(void)
 {
 	unsigned int cpuid_1_eax;
 
+	/*
+	 * Can't use microcode_loader_disabled() here - .init section
+	 * hell. It doesn't have to either - the BSP variant must've
+	 * parsed cmdline already anyway.
+	 */
 	if (dis_ucode_ldr)
 		return;
 
@@ -810,7 +820,7 @@ static int __init microcode_init(void)
 	struct cpuinfo_x86 *c = &boot_cpu_data;
 	int error;
 
-	if (dis_ucode_ldr)
+	if (microcode_loader_disabled())
 		return -EINVAL;
 
 	if (c->x86_vendor == X86_VENDOR_INTEL)

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

@@ -389,7 +389,7 @@ static int __init save_builtin_microcode(void)
 	if (xchg(&ucode_patch_va, NULL) != UCODE_BSP_LOADED)
 		return 0;
 
-	if (dis_ucode_ldr || boot_cpu_data.x86_vendor != X86_VENDOR_INTEL)
+	if (microcode_loader_disabled() || boot_cpu_data.x86_vendor != X86_VENDOR_INTEL)
 		return 0;
 
 	uci.mc = get_microcode_blob(&uci, true);

arch/x86/kernel/cpu/microcode/internal.h

@@ -94,7 +94,6 @@ static inline unsigned int x86_cpuid_family(void)
 	return x86_family(eax);
 }
 
-extern bool dis_ucode_ldr;
 extern bool force_minrev;
 
 #ifdef CONFIG_CPU_SUP_AMD

arch/x86/kernel/head32.c

@@ -145,10 +145,6 @@ void __init __no_stack_protector mk_early_pgtbl_32(void)
 	*ptr = (unsigned long)ptep + PAGE_OFFSET;
 
 #ifdef CONFIG_MICROCODE_INITRD32
-	/* Running on a hypervisor? */
-	if (native_cpuid_ecx(1) & BIT(31))
-		return;
-
 	params = (struct boot_params *)__pa_nodebug(&boot_params);
 	if (!params->hdr.ramdisk_size || !params->hdr.ramdisk_image)
 		return;