linux-kernel - [PATCH tip] x86/microcode/AMD: Read from MSR_AMD64_PATCH_LEVEL to get base

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Message-ID: <20251117191527.1487774-1-longman@redhat.com>
Date: Mon, 17 Nov 2025 14:15:27 -0500
From: Waiman Long <longman@...hat.com>
To: Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>
Cc: x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Waiman Long <longman@...hat.com>
Subject: [PATCH tip] x86/microcode/AMD: Read from MSR_AMD64_PATCH_LEVEL to get base_rev if not defined

AMD microcode is being handled inconsistenly depending on whether the
CONFIG_MICROCODE_DBG is on or off. Currently, the MSR_AMD64_PATCH_LEVEL
MSR is being read in get_patch_level() to retrieve microcode revision
when CONFIG_MICROCODE_DBG is off, but cpuid_to_ucode_rev(bsp_cpuid_1_eax)
is used to get the revision with CONFIG_MICROCODE_DBG on if
microcode_rev[cpu] hasn't been defined yet.

On a test Genoa system with a relatively new BIOS/firmware,
get_patch_level() returns 0xa101100 with cpuid_to_ucode_rev(), but
0x0a101158 when reading from MSR_AMD64_PATCH_LEVEL. It does look
like bsp_cpuid_1_eax doesn't contain the right microcode revision
for microcode embedded in the BIOS/firmware. This is problematic as
need_sha_check() may return an incorrect result.

To fix the inconsistency and provide a more correct result, always read
from the MSR in get_patch_level() to get the microcode revision number.

Fixes: 43181a47263d ("x86/microcode: Add microcode loader debugging functionality")
Signed-off-by: Waiman Long <longman@...hat.com>
---
 arch/x86/kernel/cpu/microcode/amd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/microcode/amd.c b/arch/x86/kernel/cpu/microcode/amd.c
index 3821a985f4ff..561630c017e2 100644
--- a/arch/x86/kernel/cpu/microcode/amd.c
+++ b/arch/x86/kernel/cpu/microcode/amd.c
@@ -327,7 +327,7 @@ static u32 get_patch_level(void)

 		if (!microcode_rev[cpu]) {
 			if (!base_rev)
-				base_rev = cpuid_to_ucode_rev(bsp_cpuid_1_eax);
+				native_rdmsr(MSR_AMD64_PATCH_LEVEL, base_rev, dummy);

 			microcode_rev[cpu] = base_rev;

-- 
2.51.1