linux-kernel - [PATCH v5 13/35] x86/cpu: Use parsed CPUID(0x80000000)

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Message-ID: <20250905121515.192792-14-darwi@linutronix.de>
Date: Fri,  5 Sep 2025 14:14:53 +0200
From: "Ahmed S. Darwish" <darwi@...utronix.de>
To: Borislav Petkov <bp@...en8.de>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Thomas Gleixner <tglx@...utronix.de>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Sean Christopherson <seanjc@...gle.com>,
	David Woodhouse <dwmw2@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Sohil Mehta <sohil.mehta@...el.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 v5 13/35] x86/cpu: Use parsed CPUID(0x80000000)

At the early boot code, use parsed CPUID(0x80000000) access instead of a
direct CPUID query.

Note that the modified code has the check:

    extended_cpuid_level = ((eax & 0xffff0000) == 0x80000000) ? eax : 0;

to protect against Intel 32-bit machines without an extended range, where
a CPUID(0x80000000) query will repeat the max-valid standard CPUID leaf
output.  A similar check is already done at the CPUID parser's own
CPUID(0x80000000) code:

    if ((l->max_ext_leaf & 0xffff0000) != 0x80000000) {
        // Handle error
    }

Thus, for the modified call-site code, the parsed CPUID access NULL
check:

    el0 = cpuid_leaf(c, 0x80000000);
    extended_cpuid_level = el0 ? el0->max_ext_leaf : 0;

is sufficient.

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 f989c8099490..0a0340a7ac1c 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 = cpuid_leaf(c, 0x80000000);
+	c->extended_cpuid_level = el0 ? el0->max_ext_leaf : 0;
 
 	if (c->extended_cpuid_level >= 0x80000001) {
 		cpuid(0x80000001, &eax, &ebx, &ecx, &edx);
-- 
2.50.1