Re: [PATCH v2] x86/cpu: Skip reading MSR_IA32_PLATFORM_ID in virtualized environment

[Borislav Petkov]

On Tue, May 12, 2026 at 09:57:58AM +0800, Binbin Wu wrote:
> hypervisor_present could be uninitialized if dis_ucode_ldr is true.
> intel_get_platform_id() is also called during the normal cpu initialization.

Right, that needs more surgery. See if the below works instead pls.

We might as well do it - the question whether we run on a HV comes very often
recenly - might as well make it an "official" variable.

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 10b5355b323e..67dd932305db 100644
--- a/arch/x86/include/asm/processor.h
+++ b/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);
 
diff --git a/arch/x86/kernel/cpu/microcode/amd.c b/arch/x86/kernel/cpu/microcode/amd.c
index e533881284a1..5c0afae75e9f 100644
--- a/arch/x86/kernel/cpu/microcode/amd.c
+++ b/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 */
diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index 68a1a893246c..f6722c9618e0 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/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 set
 	 *    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;
 
diff --git a/arch/x86/kernel/cpu/microcode/intel.c b/arch/x86/kernel/cpu/microcode/intel.c
index 37ac4afe0972..a4c0a0cf928b 100644
--- a/arch/x86/kernel/cpu/microcode/intel.c
+++ b/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.
diff --git a/arch/x86/kernel/cpu/microcode/internal.h b/arch/x86/kernel/cpu/microcode/internal.h
index 3b93c0676b4f..a10b547eda1e 100644
--- a/arch/x86/kernel/cpu/microcode/internal.h
+++ b/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);
 
-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette