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Date: Fri, 23 Jun 2006 09:53:11 -0400
From: Chuck Ebbert <76306.1226@compuserve.com>
Subject: [RFC, patch] i386: vgetcpu() with NUMA support
To: linux-kernel <linux-kernel@vger.kernel.org>
Cc: Andi Kleen <ak@suse.de>, Rohit Seth <rohitseth@google.com>,
       Andrew Morton <akpm@osdl.org>, Linus Torvalds <torvalds@osdl.org>,
       Martin Bligh <mbligh@google.com>
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This is attempt #3 at vgetcpu() support for i386.  It uses a GDT entry
to hold cpu and node number for fast userspace access.

changes since #2:
   added NUMA support (compile tested only, please test)
   speed improvement
   proper #define for GDT entry number
   general cleanup & documentation

to-do:
   CFI annotations

Test program:

/* vgetcpu.c: test vgetcpu()
 * boot kernel with vgetcpu patch first, then build with:
 *  gcc -O3 -o vgetcpu vgetcpu.c <srcpath>/arch/i386/kernel/vsyscall-int80.so
 * (don't forget the optimization (-O3))
 */
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>

extern int __vgetcpu(void);

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

int main(int argc, char * const argv[])
{
	long long tsc1, tsc2;
	int i, iters = 999999;
	
	i = __vgetcpu();
	printf("node: %d, cpu: %d\nBenchmarking...\n", i >> 8, i & 0xff);

	rdtscll(tsc1);
	for (i = 0; i < iters; i++)
		__vgetcpu();
	rdtscll(tsc2);

	printf("vgetcpu() took %llu clocks per call\n", (tsc2 - tsc1) / iters);

	return 0;
}

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

 arch/i386/kernel/cpu/common.c        |    3 +++
 arch/i386/kernel/head.S              |    8 +++++++-
 arch/i386/kernel/smpboot.c           |    2 ++
 arch/i386/kernel/vsyscall-getcpu.S   |   26 ++++++++++++++++++++++++++
 arch/i386/kernel/vsyscall-int80.S    |    2 ++
 arch/i386/kernel/vsyscall-sysenter.S |    2 ++
 arch/i386/kernel/vsyscall.lds.S      |    1 +
 include/asm-i386/segment.h           |    4 +++-
 8 files changed, 46 insertions(+), 2 deletions(-)

--- 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 */
+ 	gdt[GDT_ENTRY_VGETCPU].a |= 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 */
--- /dev/null
+++ 2.6.17-32/arch/i386/kernel/vsyscall-getcpu.S
@@ -0,0 +1,26 @@
+/*
+ * __vgetcpu()
+ *
+ * This file is #include'd by vsyscall-*.S to place vgetcpu after the
+ * sigreturn code.
+ *
+ * Returns logical CPU number in bits 0-7, node ID in bits 8-15;
+ * -EFAULT on error.
+ */
+
+#include <linux/errno.h>
+#include <asm/segment.h>
+
+	.text
+	.org __kernel_rt_sigreturn+32,0x90
+	.globl __vgetcpu
+	.type __vgetcpu,@function
+__vgetcpu:
+.LSTART_vgetcpu:
+	movl $((GDT_ENTRY_VGETCPU<<3)|3),%edx
+	movl $-EFAULT,%eax
+	lsll %edx,%eax
+	ret
+.LEND_vgetcpu:
+	.size __vgetcpu,.-.LSTART_vgetcpu
+
--- 2.6.17-32.orig/arch/i386/kernel/vsyscall-int80.S
+++ 2.6.17-32/arch/i386/kernel/vsyscall-int80.S
@@ -51,3 +51,5 @@ __kernel_vsyscall:
  * Get the common code for the sigreturn entry points.
  */
 #include "vsyscall-sigreturn.S"
+
+#include "vsyscall-getcpu.S"
--- 2.6.17-32.orig/arch/i386/kernel/vsyscall-sysenter.S
+++ 2.6.17-32/arch/i386/kernel/vsyscall-sysenter.S
@@ -120,3 +120,5 @@ SYSENTER_RETURN:
  * Get the common code for the sigreturn entry points.
  */
 #include "vsyscall-sigreturn.S"
+
+#include "vsyscall-getcpu.S"
--- 2.6.17-32.orig/arch/i386/kernel/vsyscall.lds.S
+++ 2.6.17-32/arch/i386/kernel/vsyscall.lds.S
@@ -57,6 +57,7 @@ VERSION
     	__kernel_vsyscall;
     	__kernel_sigreturn;
     	__kernel_rt_sigreturn;
+	__vgetcpu;
 
     local: *;
   };
--- 2.6.17-32.orig/arch/i386/kernel/smpboot.c
+++ 2.6.17-32/arch/i386/kernel/smpboot.c
@@ -615,6 +615,7 @@ static inline void map_cpu_to_node(int c
 	printk("Mapping cpu %d to node %d\n", cpu, node);
 	cpu_set(cpu, node_2_cpu_mask[node]);
 	cpu_2_node[cpu] = node;
+ 	get_cpu_gdt_table(cpu)[GDT_ENTRY_VGETCPU].a |= (node & 0xff) << 8;
 }
 
 /* undo a mapping between cpu and node. */
@@ -626,6 +627,7 @@ static inline void unmap_cpu_to_node(int
 	for (node = 0; node < MAX_NUMNODES; node ++)
 		cpu_clear(cpu, node_2_cpu_mask[node]);
 	cpu_2_node[cpu] = 0;
+ 	get_cpu_gdt_table(cpu)[GDT_ENTRY_VGETCPU].a &= ~(0xff << 8);
 }
 #else /* !CONFIG_NUMA */
 
--- 2.6.17-32.orig/include/asm-i386/segment.h
+++ 2.6.17-32/include/asm-i386/segment.h
@@ -39,7 +39,7 @@
  *  25 - APM BIOS support 
  *
  *  26 - ESPFIX small SS
- *  27 - unused
+ *  27 - vgetcpu() data
  *  28 - unused
  *  29 - unused
  *  30 - unused
@@ -74,6 +74,8 @@
 #define GDT_ENTRY_ESPFIX_SS		(GDT_ENTRY_KERNEL_BASE + 14)
 #define __ESPFIX_SS (GDT_ENTRY_ESPFIX_SS * 8)
 
+#define GDT_ENTRY_VGETCPU		(GDT_ENTRY_KERNEL_BASE + 15)
+
 #define GDT_ENTRY_DOUBLEFAULT_TSS	31
 
 /*
-- 
Chuck
 "You can't read a newspaper if you can't read."  --George W. Bush