linux-kernel - [PATCH] x86/microcode/AMD: Allow loader debugging to be enabled on baremetal too

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Message-ID: <20260108165028.27417-1-bp@kernel.org>
Date: Thu,  8 Jan 2026 17:50:28 +0100
From: Borislav Petkov <bp@...nel.org>
To: X86 ML <x86@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
	"Borislav Petkov (AMD)" <bp@...en8.de>
Subject: [PATCH] x86/microcode/AMD: Allow loader debugging to be enabled on baremetal too

From: "Borislav Petkov (AMD)" <bp@...en8.de>

Debugging the loader on baremetal does make sense, so enable it there
too.

Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
---
 arch/x86/Kconfig                         |  8 +++++---
 arch/x86/kernel/cpu/microcode/amd.c      |  4 ++--
 arch/x86/kernel/cpu/microcode/core.c     | 16 ++++++++++++----
 arch/x86/kernel/cpu/microcode/internal.h |  1 +
 4 files changed, 20 insertions(+), 9 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 80527299f859..c10593768984 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1366,10 +1366,12 @@ config MICROCODE_DBG
 	default n
 	depends on MICROCODE
 	help
-	  Enable code which allows for debugging the microcode loader in
-	  a guest. Meaning the patch loading is simulated but everything else
+	  Enable code which allows to debug the microcode loader. When running
+	  in a guest the patch loading is simulated but everything else
 	  related to patch parsing and handling is done as on baremetal with
-	  the purpose of debugging solely the software side of things.
+	  the purpose of debugging solely the software side of things. On
+	  baremetal, it simply dumps additional debugging information during
+	  normal operation.
 
 	  You almost certainly want to say n here.
 
diff --git a/arch/x86/kernel/cpu/microcode/amd.c b/arch/x86/kernel/cpu/microcode/amd.c
index 46673530bc6f..caa0f595abcf 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)) {
+	if (IS_ENABLED(CONFIG_MICROCODE_DBG) && 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))
+	if (IS_ENABLED(CONFIG_MICROCODE_DBG) && 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 68049f171860..651202e6fefb 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -57,6 +57,8 @@ bool force_minrev = IS_ENABLED(CONFIG_MICROCODE_LATE_FORCE_MINREV);
 u32 base_rev;
 u32 microcode_rev[NR_CPUS] = {};
 
+bool hypervisor_present;
+
 /*
  * Synchronization.
  *
@@ -117,7 +119,13 @@ 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;
+	}
+
+	/*
 	 * 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
@@ -127,9 +135,9 @@ bool __init microcode_loader_disabled(void)
 	 * 3) Certain AMD patch levels are not allowed to be
 	 *    overwritten.
 	 */
-	if (!cpuid_feature() ||
-	    ((native_cpuid_ecx(1) & BIT(31)) &&
-	      !IS_ENABLED(CONFIG_MICROCODE_DBG)) ||
+	hypervisor_present = native_cpuid_ecx(1) & BIT(31);
+
+	if ((hypervisor_present && !IS_ENABLED(CONFIG_MICROCODE_DBG)) ||
 	    amd_check_current_patch_level())
 		dis_ucode_ldr = true;
 
diff --git a/arch/x86/kernel/cpu/microcode/internal.h b/arch/x86/kernel/cpu/microcode/internal.h
index a10b547eda1e..3b93c0676b4f 100644
--- a/arch/x86/kernel/cpu/microcode/internal.h
+++ b/arch/x86/kernel/cpu/microcode/internal.h
@@ -48,6 +48,7 @@ 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);
 
-- 
2.51.0