[RFC, patch] i386: vgetcpu()

[Chuck Ebbert <76306.1226@compuserve.com>]

Use the limit field of a GDT entry to store the current CPU
number for fast userspace access.  This still leaves 12 bits
free for other information.

/* vgetcpu.c: get CPU number we are running on.
 * Prints -1 if error occurred.
 */
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char * const argv[])
{
	int cpu;

	asm (
		"	lsl %1,%0		\n" \
		"	jnz 2f			\n" \
		"	and $0xff,%0		\n" \
		"2:				\n" \
		: "=&r" (cpu)
		: "r" ((27 << 3) | 3), "0" (-1)
	);

	printf("cpu: %d\n", cpu);

	return 0;
}

Signed-off-by: Chuck Ebbert <76306.1226@compuserve.com>

--- 2.6.17-32.orig/arch/i386/kernel/cpu/common.c
+++ 2.6.17-32/arch/i386/kernel/cpu/common.c
@@ -642,6 +642,9 @@ void __cpuinit cpu_init(void)
 		((((__u64)stk16_off) << 32) & 0xff00000000000000ULL) |
 		(CPU_16BIT_STACK_SIZE - 1);
 
+	/* Set up GDT entry for per-cpu data */
+ 	*(__u64 *)(&gdt[27]) |= cpu;
+
 	cpu_gdt_descr->size = GDT_SIZE - 1;
  	cpu_gdt_descr->address = (unsigned long)gdt;
 
--- 2.6.17-32.orig/arch/i386/kernel/head.S
+++ 2.6.17-32/arch/i386/kernel/head.S
@@ -525,7 +525,13 @@ ENTRY(cpu_gdt_table)
 	.quad 0x004092000000ffff	/* 0xc8 APM DS    data */
 
 	.quad 0x0000920000000000	/* 0xd0 - ESPFIX 16-bit SS */
-	.quad 0x0000000000000000	/* 0xd8 - unused */
+
+	/*
+	 * Use a GDT entry to store per-cpu data for user space (DPL 3.)
+	 * 32-bit data segment, byte granularity, base 0, limit set at runtime.
+	 */
+	.quad 0x0040f20000000000	/* 0xd8 - for per-cpu user data */
+
 	.quad 0x0000000000000000	/* 0xe0 - unused */
 	.quad 0x0000000000000000	/* 0xe8 - unused */
 	.quad 0x0000000000000000	/* 0xf0 - unused */
-- 
Chuck
 "You can't read a newspaper if you can't read."  --George W. Bush