linux-kernel - [PATCH] x86/CPU/AMD: Simplify the spectral chicken fix

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Message-ID: <20251230110731.28108-1-bp@kernel.org>
Date: Tue, 30 Dec 2025 12:07:31 +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/CPU/AMD: Simplify the spectral chicken fix

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

msr_set_bit() takes a bit number to set but MSR_ZEN2_SPECTRAL_CHICKEN_BIT
is a bit mask. The usual pattern that code uses is a _BIT-named type
macro instead of a mask.

So convert it to a bit number to reflect that.

Also, msr_set_bit() already does the reading and checking whether the
bit needs to be set so use that instead of a local variable.

Fixup tabbing while at it.

No functional changes.

Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
---
 arch/x86/include/asm/msr-index.h       |  4 ++--
 arch/x86/kernel/cpu/amd.c              | 10 ++--------
 tools/arch/x86/include/asm/msr-index.h |  4 ++--
 3 files changed, 6 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index 3d0a0950d20a..43adc38d31d5 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -794,8 +794,8 @@
 #define MSR_F19H_UMC_PERF_CTR           0xc0010801
 
 /* Zen 2 */
-#define MSR_ZEN2_SPECTRAL_CHICKEN       0xc00110e3
-#define MSR_ZEN2_SPECTRAL_CHICKEN_BIT   BIT_ULL(1)
+#define MSR_ZEN2_SPECTRAL_CHICKEN	0xc00110e3
+#define MSR_ZEN2_SPECTRAL_CHICKEN_BIT	1
 
 /* Fam 17h MSRs */
 #define MSR_F17H_IRPERF			0xc00000e9
diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index c792c2afd849..09de584e4c8f 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -900,20 +900,14 @@ static void fix_erratum_1386(struct cpuinfo_x86 *c)
 void init_spectral_chicken(struct cpuinfo_x86 *c)
 {
 #ifdef CONFIG_MITIGATION_UNRET_ENTRY
-	u64 value;
-
 	/*
 	 * On Zen2 we offer this chicken (bit) on the altar of Speculation.
 	 *
 	 * This suppresses speculation from the middle of a basic block, i.e. it
 	 * suppresses non-branch predictions.
 	 */
-	if (!cpu_has(c, X86_FEATURE_HYPERVISOR)) {
-		if (!rdmsrq_safe(MSR_ZEN2_SPECTRAL_CHICKEN, &value)) {
-			value |= MSR_ZEN2_SPECTRAL_CHICKEN_BIT;
-			wrmsrq_safe(MSR_ZEN2_SPECTRAL_CHICKEN, value);
-		}
-	}
+	if (!cpu_has(c, X86_FEATURE_HYPERVISOR))
+		msr_set_bit(MSR_ZEN2_SPECTRAL_CHICKEN, MSR_ZEN2_SPECTRAL_CHICKEN_BIT);
 #endif
 }
 
diff --git a/tools/arch/x86/include/asm/msr-index.h b/tools/arch/x86/include/asm/msr-index.h
index 9e1720d73244..d4137a302793 100644
--- a/tools/arch/x86/include/asm/msr-index.h
+++ b/tools/arch/x86/include/asm/msr-index.h
@@ -770,8 +770,8 @@
 #define MSR_F19H_UMC_PERF_CTR           0xc0010801
 
 /* Zen 2 */
-#define MSR_ZEN2_SPECTRAL_CHICKEN       0xc00110e3
-#define MSR_ZEN2_SPECTRAL_CHICKEN_BIT   BIT_ULL(1)
+#define MSR_ZEN2_SPECTRAL_CHICKEN	0xc00110e3
+#define MSR_ZEN2_SPECTRAL_CHICKEN_BIT	1
 
 /* Fam 17h MSRs */
 #define MSR_F17H_IRPERF			0xc00000e9
-- 
2.51.0