linux-kernel - [PATCH v5 12/35] x86/cpuid: Parse CPUID(0x80000000)

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Message-ID: <20250905121515.192792-13-darwi@linutronix.de>
Date: Fri,  5 Sep 2025 14:14:52 +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 12/35] x86/cpuid: Parse CPUID(0x80000000)

Add CPUID parser logic for CPUID(0x80000000).

Verify the query output beforehand, since legacy 32-bit Intel machines
without an extended range will just repeat the output of the highest
standard CPUID leaf available.  This is similar to what is done at
arch/x86/kernel/head_32.S and arch/x86/kernel/cpu/common.c.

References: 8a50e5135af0 ("x86-32: Use symbolic constants, safer CPUID when enabling EFER.NX")
References: 67ad24e6d39c ("- pre5:    - Rasmus Andersen: add proper...")  # Historical git
Signed-off-by: Ahmed S. Darwish <darwi@...utronix.de>
Cc: "H. Peter Anvin" <hpa@...or.com>
Link: https://lore.kernel.org/r/d4fcfd91-cc92-4b3c-9dd2-56ecd754cecc@citrix.com
---
 arch/x86/include/asm/cpuid/types.h |  7 ++++++-
 arch/x86/kernel/cpu/cpuid_parser.c | 22 +++++++++++++++++++++-
 arch/x86/kernel/cpu/cpuid_parser.h |  1 +
 3 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/cpuid/types.h b/arch/x86/include/asm/cpuid/types.h
index 9b8bd6b22ff3..e57245ca6419 100644
--- a/arch/x86/include/asm/cpuid/types.h
+++ b/arch/x86/include/asm/cpuid/types.h
@@ -33,7 +33,11 @@ enum cpuid_regs_idx {
 #define CPUID_LEAF_TILE		0x1d
 
 #define CPUID_BASE_START	0x0
-#define CPUID_BASE_END		(CPUID_BASE_START + 0xffff)
+#define CPUID_EXT_START		0x80000000
+
+#define __CPUID_RANGE_END(idx)	((idx) + 0xffff)
+#define CPUID_BASE_END		__CPUID_RANGE_END(CPUID_BASE_START)
+#define CPUID_EXT_END		__CPUID_RANGE_END(CPUID_EXT_START)
 
 /*
  * Types for CPUID(0x2) parsing:
@@ -232,6 +236,7 @@ struct cpuid_leaves {
 	/*		Leaf		Subleaf number (or max number of dynamic subleaves) */
 	CPUID_LEAF   (	0x0,		0  );
 	CPUID_LEAF   (	0x1,		0  );
+	CPUID_LEAF   (  0x80000000,	0  );
 };
 
 /*
diff --git a/arch/x86/kernel/cpu/cpuid_parser.c b/arch/x86/kernel/cpu/cpuid_parser.c
index 15264696eaad..9c40c180e3d4 100644
--- a/arch/x86/kernel/cpu/cpuid_parser.c
+++ b/arch/x86/kernel/cpu/cpuid_parser.c
@@ -27,6 +27,23 @@ static void cpuid_read_generic(const struct cpuid_parse_entry *e, struct cpuid_r
 		cpuid_read_subleaf(e->leaf, e->subleaf + i, output->regs);
 }
 
+static void cpuid_read_0x80000000(const struct cpuid_parse_entry *e, struct cpuid_read_output *output)
+{
+	struct leaf_0x80000000_0 *el0 = (struct leaf_0x80000000_0 *)output->regs;
+
+	cpuid_read_subleaf(e->leaf, e->subleaf, el0);
+
+	/*
+	 * Protect against Intel 32-bit CPUs lacking an extended CPUID range. A
+	 * CPUID(0x80000000) query on such machines will just repeat the output
+	 * of the highest standard CPUID leaf.
+	 */
+	if ((el0->max_ext_leaf & 0xffff0000) != 0x80000000)
+		return;
+
+	output->info->nr_entries = 1;
+}
+
 /*
  * CPUID parser tables:
  *
@@ -45,9 +62,11 @@ static const struct cpuid_parse_entry cpuid_parse_entries[] = {
 static unsigned int cpuid_range_max_leaf(const struct cpuid_table *t, unsigned int range)
 {
 	const struct leaf_0x0_0 *l0 = __cpuid_table_subleaf(t, 0x0, 0);
+	const struct leaf_0x80000000_0 *el0 = __cpuid_table_subleaf(t, 0x80000000, 0);
 
 	switch (range) {
 	case CPUID_BASE_START:	return l0  ?  l0->max_std_leaf : 0;
+	case CPUID_EXT_START:	return el0 ? el0->max_ext_leaf : 0;
 	default:		return 0;
 	}
 }
@@ -63,7 +82,8 @@ cpuid_range_valid(const struct cpuid_table *t, unsigned int leaf, unsigned int s
 
 static bool cpuid_leaf_in_range(const struct cpuid_table *t, unsigned int leaf)
 {
-	return cpuid_range_valid(t, leaf, CPUID_BASE_START, CPUID_BASE_END);
+	return cpuid_range_valid(t, leaf, CPUID_BASE_START, CPUID_BASE_END) ||
+	       cpuid_range_valid(t, leaf, CPUID_EXT_START, CPUID_EXT_END);
 }
 
 static void
diff --git a/arch/x86/kernel/cpu/cpuid_parser.h b/arch/x86/kernel/cpu/cpuid_parser.h
index acddcbc9bd06..39a361eed7ce 100644
--- a/arch/x86/kernel/cpu/cpuid_parser.h
+++ b/arch/x86/kernel/cpu/cpuid_parser.h
@@ -111,5 +111,6 @@ struct cpuid_parse_entry {
 	/*			Leaf		Subleaf		Reader function */		\
 	CPUID_PARSE_ENTRY   (	0x0,		0,		generic			),	\
 	CPUID_PARSE_ENTRY   (	0x1,		0,		generic			),	\
+	CPUID_PARSE_ENTRY   (	0x80000000,	0,		0x80000000		),	\
 
 #endif /* _ARCH_X86_CPUID_PARSER_H */
-- 
2.50.1