linux-kernel - [PATCH 1/3] x86: Provide a full setup for getcpu on x86-32.

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Message-Id: <20221125094216.3663444-2-bigeasy@linutronix.de>
Date:   Fri, 25 Nov 2022 10:42:14 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-kernel@...r.kernel.org, x86@...nel.org
Cc:     Roland Mainz <roland.mainz@...bsig.org>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH 1/3] x86: Provide a full setup for getcpu on x86-32.

setup_getcpu() configures two things:
- writes the current CPU & node information into the MSR_TSC_AUX.
- writes the same information as a GDT entry.

By using the "full" setup_getcpu() on i386 it is possible to read the
CPU information in userland via rdtscp.

Provide an GDT_ENTRY_CPUNODE for x86-32.
Move the CONFIG_X86_64 so the x86-64 implementation becomes also
available for x86-32.

Reviewed-By: Roland Mainz <roland.mainz@...bsig.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 arch/x86/include/asm/segment.h | 8 ++++----
 arch/x86/kernel/cpu/common.c   | 4 +---
 2 files changed, 5 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/segment.h b/arch/x86/include/asm/segment.h
index 2e7890dd58a47..662281c2a907c 100644
--- a/arch/x86/include/asm/segment.h
+++ b/arch/x86/include/asm/segment.h
@@ -96,7 +96,7 @@
  *
  *  26 - ESPFIX small SS
  *  27 - per-cpu			[ offset to per-cpu data area ]
- *  28 - unused
+ *  28 - VDSO getcpu
  *  29 - unused
  *  30 - unused
  *  31 - TSS for double fault handler
@@ -119,6 +119,7 @@
 
 #define GDT_ENTRY_ESPFIX_SS		26
 #define GDT_ENTRY_PERCPU		27
+#define GDT_ENTRY_CPUNODE		28
 
 #define GDT_ENTRY_DOUBLEFAULT_TSS	31
 
@@ -158,6 +159,8 @@
 # define __KERNEL_PERCPU		0
 #endif
 
+#define __CPUNODE_SEG			(GDT_ENTRY_CPUNODE*8 + 3)
+
 #else /* 64-bit: */
 
 #include <asm/cache.h>
@@ -226,8 +229,6 @@
 #define GDT_ENTRY_TLS_ENTRIES		3
 #define TLS_SIZE			(GDT_ENTRY_TLS_ENTRIES* 8)
 
-#ifdef CONFIG_X86_64
-
 /* Bit size and mask of CPU number stored in the per CPU data (and TSC_AUX) */
 #define VDSO_CPUNODE_BITS		12
 #define VDSO_CPUNODE_MASK		0xfff
@@ -265,7 +266,6 @@ static inline void vdso_read_cpunode(unsigned *cpu, unsigned *node)
 }
 
 #endif /* !__ASSEMBLY__ */
-#endif /* CONFIG_X86_64 */
 
 #ifdef __KERNEL__
 
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 3e508f2390983..520a0a2e765d3 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -2128,7 +2128,6 @@ static void wait_for_master_cpu(int cpu)
 #endif
 }
 
-#ifdef CONFIG_X86_64
 static inline void setup_getcpu(int cpu)
 {
 	unsigned long cpudata = vdso_encode_cpunode(cpu, early_cpu_to_node(cpu));
@@ -2150,6 +2149,7 @@ static inline void setup_getcpu(int cpu)
 	write_gdt_entry(get_cpu_gdt_rw(cpu), GDT_ENTRY_CPUNODE, &d, DESCTYPE_S);
 }
 
+#ifdef CONFIG_X86_64
 static inline void ucode_cpu_init(int cpu)
 {
 	if (cpu)
@@ -2169,8 +2169,6 @@ static inline void tss_setup_ist(struct tss_struct *tss)
 
 #else /* CONFIG_X86_64 */
 
-static inline void setup_getcpu(int cpu) { }
-
 static inline void ucode_cpu_init(int cpu)
 {
 	show_ucode_info_early();
-- 
2.38.1