[RFC, patch] i386: vgetcpu()

[RFC, patch] i386: vgetcpu()

[Chuck Ebbert]

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

Re: [RFC, patch] i386: vgetcpu()

[Andi Kleen]

On Tuesday 20 June 2006 22:25, Chuck Ebbert wrote:
> 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.

Nice trick. Maybe I'll even add that to the x86-64 implementation
if it's fast enough. Do you have numbers?

But it needs to be encapsulated in a wrapper I think. Just exposing
it to user space is the wrong way to do this.

-Andi

Re: [RFC, patch] i386: vgetcpu()

[Chuck Ebbert]

In-Reply-To: <200606202257.16033.ak@suse.de>

On Tue, 20 Jun 2006 22:57:16 +0200, Andi Kleen wrote:

> On Tuesday 20 June 2006 22:25, Chuck Ebbert wrote:
> > 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.
> 
> Nice trick. Maybe I'll even add that to the x86-64 implementation
> if it's fast enough. Do you have numbers?
> 

I got ~13 clocks on x86_64 and 21 on PII.  Test program below.

> But it needs to be encapsulated in a wrapper I think. Just exposing
> it to user space is the wrong way to do this.

Well there's no real way to hide it, but something more is definitely
needed.  A new vdso entry point is easy but how do you tell userspace
it's there?  Does glibc look at the kernel version in the note?


/* test how fast lsl, test for success + mask result runs
 * leave DO_TEST undefined to measure overhead
 */
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>

#define rdtscll(t)	asm volatile ("rdtsc" : "=A" (t))

#ifndef ITERS
#define ITERS	1000000
#endif

int main(int argc, char * const argv[])
{
	unsigned long long tsc1, tsc2;
	int count, cpu, junk;

	rdtscll(tsc1);
	asm (
		"	pushl %%ds		\n"
		"	popl %2			\n"
		"1:				\n"
#ifdef DO_TEST
		"	lsl %2,%0		\n"
		"	jnz 2f			\n"
		"	and $0xff,%0		\n"
#endif
		"	dec %1			\n"
		"	jnz 1b			\n"
		"2:				\n"
		: "=&r" (cpu), "=&r" (count), "=&r" (junk)
		: "1" (ITERS), "0" (-1)
	);
	rdtscll(tsc2);

	if (count == 0)
		printf("loops: %d, avg: %llu clocks\n",
			ITERS, (tsc2 - tsc1) / ITERS);
	return 0;
}
-- 
Chuck
 "You can't read a newspaper if you can't read."  --George W. Bush