x86/microcode: Do not access MSR_IA32_PLATFORM_ID when running as a guest

Patch in Fixes: causes the usual:

  unchecked MSR access error: RDMSR from 0x17 at ... (intel_get_platform_id)
  Call Trace:
   early_init_intel
   early_cpu_init
   setup_arch
   _printk
   start_kernel
   x86_64_start_reservations
   x86_64_start_kernel
   common_startup_64

because the kernel is booted in a guest.

In order to avoid it, this MSR access needs to be prevented when running
virtualized. That is usually done by checking X86_FEATURE_HYPERVISOR but
for this particular case it is too early yet.

The platform ID needs to be read as early as when microcode is loaded on
the BSP:

  load_ucode_bsp ... -> get_microcode_blob ... -> intel_find_matching_signature

and by that time, CPUID leafs haven't been parsed yet.

The microcode loader already has logic to check early whether the kernel
is running virtualized so make that globally available to arch/x86/. The
query whether running virtualized is getting more and more prominent in
recent times so might as well make it an arch-global var which the rest
of the code can use.

Fixes: d8630b6 ("x86/cpu: Add platform ID to CPU info structure")
Reported-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Tested-by: Binbin Wu <binbin.wu@linux.intel.com>
Link: https://lore.kernel.org/all/20260430020953.1405535-1-binbin.wu@linux.intel.com

Author: bp3tk0v

arch/x86/include/asm/processor.h

@@ -733,6 +733,7 @@ bool xen_set_default_idle(void);
 #endif
 
 void __noreturn stop_this_cpu(void *dummy);
+extern bool x86_hypervisor_present;
 void microcode_check(struct cpuinfo_x86 *prev_info);
 void store_cpu_caps(struct cpuinfo_x86 *info);
 

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

@@ -322,7 +322,7 @@ static u32 get_patch_level(void)
 {
 	u32 rev, dummy __always_unused;
 
-	if (IS_ENABLED(CONFIG_MICROCODE_DBG) && hypervisor_present) {
+	if (IS_ENABLED(CONFIG_MICROCODE_DBG) && x86_hypervisor_present) {
 		int cpu = smp_processor_id();
 
 		if (!microcode_rev[cpu]) {
@@ -714,7 +714,7 @@ static bool __apply_microcode_amd(struct microcode_amd *mc, u32 *cur_rev,
 			invlpg(p_addr_end);
 	}
 
-	if (IS_ENABLED(CONFIG_MICROCODE_DBG) && hypervisor_present)
+	if (IS_ENABLED(CONFIG_MICROCODE_DBG) && x86_hypervisor_present)
 		microcode_rev[smp_processor_id()] = mc->hdr.patch_id;
 
 	/* verify patch application was successful */

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

@@ -57,7 +57,7 @@ bool force_minrev = IS_ENABLED(CONFIG_MICROCODE_LATE_FORCE_MINREV);
 u32 base_rev;
 u32 microcode_rev[NR_CPUS] = {};
 
-bool hypervisor_present;
+bool __ro_after_init x86_hypervisor_present;
 
 /*
  * Synchronization.
@@ -118,14 +118,9 @@ bool __init microcode_loader_disabled(void)
 	/*
 	 * Disable when:
 	 *
-	 * 1) The CPU does not support CPUID.
-	 */
-	if (!cpuid_feature()) {
-		dis_ucode_ldr = true;
-		return dis_ucode_ldr;
-	}
-
-	/*
+	 * 1) The CPU does not support CPUID, detected below in
+	 *    load_ucode_bsp().
+	 *
 	 * 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
@@ -135,9 +130,7 @@ bool __init microcode_loader_disabled(void)
 	 * 3) Certain AMD patch levels are not allowed to be
 	 *    overwritten.
 	 */
-	hypervisor_present = native_cpuid_ecx(1) & BIT(31);
-
-	if ((hypervisor_present && !IS_ENABLED(CONFIG_MICROCODE_DBG)) ||
+	if ((x86_hypervisor_present && !IS_ENABLED(CONFIG_MICROCODE_DBG)) ||
 	    amd_check_current_patch_level())
 		dis_ucode_ldr = true;
 
@@ -179,6 +172,11 @@ void __init load_ucode_bsp(void)
 
 	early_parse_cmdline();
 
+	if (!cpuid_feature())
+		dis_ucode_ldr = true;
+	else
+		x86_hypervisor_present = native_cpuid_ecx(1) & BIT(31);
+
 	if (microcode_loader_disabled())
 		return;
 

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

@@ -138,6 +138,9 @@ u32 intel_get_platform_id(void)
 {
 	unsigned int val[2];
 
+	if (x86_hypervisor_present)
+		return 0;
+
 	/*
 	 * This can be called early. Use CPUID directly instead of
 	 * relying on cpuinfo_x86 which may not be fully initialized.

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

@@ -48,7 +48,6 @@ extern struct early_load_data early_data;
 extern struct ucode_cpu_info ucode_cpu_info[];
 extern u32 microcode_rev[NR_CPUS];
 extern u32 base_rev;
-extern bool hypervisor_present;
 
 struct cpio_data find_microcode_in_initrd(const char *path);