linux-kernel - [PATCH v1 10/26] x86/cpu: Use scanned CPUID(0x80000001)

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Message-ID: <20250506050437.10264-11-darwi@linutronix.de>
Date: Tue,  6 May 2025 07:04:21 +0200
From: "Ahmed S. Darwish" <darwi@...utronix.de>
To: Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Thomas Gleixner <tglx@...utronix.de>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	John Ogness <john.ogness@...utronix.de>,
	x86@...nel.org,
	x86-cpuid@...ts.linux.dev,
	LKML <linux-kernel@...r.kernel.org>,
	"Ahmed S. Darwish" <darwi@...utronix.de>
Subject: [PATCH v1 10/26] x86/cpu: Use scanned CPUID(0x80000001)

Use scanned CPUID(0x80000001) access instead of a direct CPUID query.

The affected code has the check:

	(eax & 0xffff0000) == 0x80000000

to protect against 32-bit CPUs that lack extended CPUID support.  A
similar check is already done at the CPUID(0x80000001) scanner read
function at cpuid_scanner.c:

	/*
	 * Protect against 32-bit CPUs lacking extended CPUID support: Max
	 * extended CPUID leaf must be in the 0x80000001-0x8000ffff range.
	 */
	if ((el0->max_ext_leaf & 0xffff0000) != 0x80000000) {
		// Handle error
	}

Thus, just check that the scanned CPUID macro:

	cpudata_cpuid(c, 0x80000000)

does not return NULL, thus providing a sanity check similar to the
original code.

Signed-off-by: Ahmed S. Darwish <darwi@...utronix.de>
---
 arch/x86/kernel/cpu/common.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 16086725d722..59ddf6b074f2 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -971,6 +971,7 @@ static void init_speculation_control(struct cpuinfo_x86 *c)
 
 void get_cpu_cap(struct cpuinfo_x86 *c)
 {
+	const struct leaf_0x80000000_0 *el0;
 	u32 eax, ebx, ecx, edx;
 
 	/* Intel-defined flags: level 0x00000001 */
@@ -1006,12 +1007,8 @@ void get_cpu_cap(struct cpuinfo_x86 *c)
 		c->x86_capability[CPUID_D_1_EAX] = eax;
 	}
 
-	/*
-	 * Check if extended CPUID leaves are implemented: Max extended
-	 * CPUID leaf must be in the 0x80000001-0x8000ffff range.
-	 */
-	eax = cpuid_eax(0x80000000);
-	c->extended_cpuid_level = ((eax & 0xffff0000) == 0x80000000) ? eax : 0;
+	el0 = cpudata_cpuid(c, 0x80000000);
+	c->extended_cpuid_level = (el0) ? el0->max_ext_leaf : 0;
 
 	if (c->extended_cpuid_level >= 0x80000001) {
 		cpuid(0x80000001, &eax, &ebx, &ecx, &edx);
-- 
2.49.0