x86/cpu: Provide the full setup for getcpu() on x86-32

setup_getcpu() configures two things:

  - it writes the current CPU & node information into MSR_TSC_AUX
  - it 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() or via LSL from the GDT.

Provide an GDT_ENTRY_CPUNODE for x86-32 and make the setup function
unconditionally available.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Roland Mainz <roland.mainz@nrubsig.org>
Link: https://lore.kernel.org/r/20221125094216.3663444-2-bigeasy@linutronix.de

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
 
@@ -159,6 +160,8 @@
 # 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__
 

arch/x86/kernel/cpu/common.c

@@ -2124,7 +2124,6 @@ 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));
@@ -2146,6 +2145,7 @@ 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)
@@ -2165,8 +2165,6 @@ 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();