linux-kernel - [tip: x86/cpu] x86/cpu: Remove CPUID leaf 0x2 parsing loop

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Message-ID: <174289539700.14745.1737394657121748708.tip-bot2@tip-bot2>
Date: Tue, 25 Mar 2025 09:36:36 -0000
From: "tip-bot2 for Ahmed S. Darwish" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Ingo Molnar <mingo@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
 "Ahmed S. Darwish" <darwi@...utronix.de>, "H. Peter Anvin" <hpa@...or.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: x86/cpu] x86/cpu: Remove CPUID leaf 0x2 parsing loop

The following commit has been merged into the x86/cpu branch of tip:

Commit-ID:     b5969494c8d8a2384c0636707a538efad15d8660
Gitweb:        https://git.kernel.org/tip/b5969494c8d8a2384c0636707a538efad15d8660
Author:        Ahmed S. Darwish <darwi@...utronix.de>
AuthorDate:    Mon, 24 Mar 2025 14:32:56 +01:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Tue, 25 Mar 2025 10:21:46 +01:00

x86/cpu: Remove CPUID leaf 0x2 parsing loop

Leaf 0x2 output includes a "query count" byte where it was supposed to
specify the number of repeated CPUID leaf 0x2 subleaf 0 queries needed to
extract all of the CPU's cache and TLB descriptors.

Per current Intel manuals, all CPUs supporting this leaf "will always"
return an iteration count of 1.

Remove the leaf 0x2 query loop and just query the hardware once.

Note, as previously done in:

  aec28d852ed2 ("x86/cpuid: Standardize on u32 in <asm/cpuid/api.h>")

standardize on using 'u32' and 'u8' types.

Suggested-by: Ingo Molnar <mingo@...nel.org>
Suggested-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Ahmed S. Darwish <darwi@...utronix.de>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Link: https://lore.kernel.org/r/20250324133324.23458-2-darwi@linutronix.de
---
 arch/x86/kernel/cpu/intel.c | 30 +++++++++++++++---------------
 1 file changed, 15 insertions(+), 15 deletions(-)

diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 4cbb2e6..0570d4d 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -6,6 +6,7 @@
 #include <linux/minmax.h>
 #include <linux/smp.h>
 #include <linux/string.h>
+#include <linux/types.h>
 
 #ifdef CONFIG_X86_64
 #include <linux/topology.h>
@@ -777,28 +778,27 @@ static void intel_tlb_lookup(const unsigned char desc)
 
 static void intel_detect_tlb(struct cpuinfo_x86 *c)
 {
-	int i, j, n;
-	unsigned int regs[4];
-	unsigned char *desc = (unsigned char *)regs;
+	u32 regs[4];
+	u8 *desc = (u8 *)regs;
 
 	if (c->cpuid_level < 2)
 		return;
 
-	/* Number of times to iterate */
-	n = cpuid_eax(2) & 0xFF;
+	cpuid(2, &regs[0], &regs[1], &regs[2], &regs[3]);
 
-	for (i = 0 ; i < n ; i++) {
-		cpuid(2, &regs[0], &regs[1], &regs[2], &regs[3]);
-
-		/* If bit 31 is set, this is an unknown format */
-		for (j = 0 ; j < 4 ; j++)
-			if (regs[j] & (1 << 31))
-				regs[j] = 0;
+	/* Intel CPUs must report an iteration count of 1 */
+	if (desc[0] != 0x01)
+		return;
 
-		/* Byte 0 is level count, not a descriptor */
-		for (j = 1 ; j < 16 ; j++)
-			intel_tlb_lookup(desc[j]);
+	/* If a register's bit 31 is set, it is an unknown format */
+	for (int i = 0; i < 4; i++) {
+		if (regs[i] & (1 << 31))
+			regs[i] = 0;
 	}
+
+	/* Skip the first byte as it is not a descriptor */
+	for (int i = 1; i < 16; i++)
+		intel_tlb_lookup(desc[i]);
 }
 
 static const struct cpu_dev intel_cpu_dev = {