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Subject[RFC, patch] i386: vgetcpu() for NUMA, take 2
This is attempt #2 at vgetcpu() NUMA support for i386.  It uses a
GDT entry to hold cpu and node number for fast userspace access.

changes since #1:
proper function prototype (same as x86_64)
changed alignment of vsyscall functions to 16 bytes
(sigreturn needs to stay fixed, others can move)

to-do:
CFI annotations
test NUMA on real NUMA hardware (someone please test)

Test program:

/* vgetcpu.c: test how fast vgetcpu runs
* boot kernel with vgetcpu patch first, then:
* 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 __attribute__ ((regparm(2))) __vgetcpu(int *cpu, int *node);

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

int main(int argc, char * const argv[])
{
long long tsc1, tsc2;
int i, cpu = 999, node = 999, iters = 99999;
if (__vgetcpu(&cpu, &node) || node == 999 || cpu == 999) {
printf("vgetcpu failed!\n");
_exit(1);
}
printf("node: %d, cpu: %d\n", node, cpu);
rdtscll(tsc1);
for (i = 0; i < iters; i++)
__vgetcpu(&cpu, &node);
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 | 11 +++++++-
arch/i386/kernel/smpboot.c | 2 +
arch/i386/kernel/vsyscall-getcpu.S | 42 ++++++++++++++++++++++++++++++++++
arch/i386/kernel/vsyscall-int80.S | 2 +
arch/i386/kernel/vsyscall-sigreturn.S | 3 --
arch/i386/kernel/vsyscall-sysenter.S | 2 +
arch/i386/kernel/vsyscall.lds.S | 1
include/asm-i386/segment.h | 4 ++-
9 files changed, 65 insertions(+), 5 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 & 0xff;
+
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
@@ -480,7 +480,7 @@ ENTRY(boot_gdt_table)
.quad 0x00cf92000000ffff /* kernel 4GB data at 0x00000000 */

/*
- * The Global Descriptor Table contains 28 quadwords, per-CPU.
+ * The Global Descriptor Table contains 32 quadwords, per-CPU.
*/
.align L1_CACHE_BYTES
ENTRY(cpu_gdt_table)
@@ -525,7 +525,14 @@ ENTRY(cpu_gdt_table)
.quad 0x004092000000ffff /* 0xc8 APM DS data */

.quad 0x0000920000000000 /* 0xd0 - ESPFIX 16-bit SS */
- .quad 0x0000000000000000 /* 0xd8 - unused */
+
+ /*
+ * Use GDT entries to store per-cpu data for user space (DPL 3.)
+ * 32-bit data segment, byte granularity, base 0, limit set at runtime.
+ * Userspace will use LSL to access this data, stored in the limit field.
+ */
+ .quad 0x0040f20000000000 /* 0xd8 - nodeid and logical CPU number */
+
.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 +1,42 @@
+/*
+ * fastcall int __vgetcpu(int *cpu, int *node)
+ *
+ * This file is #include'd by vsyscall-*.S to place vgetcpu after the
+ * sigreturn code.
+ *
+ * Puts logical CPU number in *cpu, node ID in *node;
+ * returns 0 for success and -EFAULT on error.
+ *
+ * CPU number and node ID are 8 bits each, with 4 total bits available
+ * for future growth of either field.
+ */
+
+#include <linux/errno.h>
+#include <asm/segment.h>
+
+ .text
+ .balign 16
+ .globl __vgetcpu
+ .type __vgetcpu,@function
+__vgetcpu:
+.LSTART_vgetcpu:
+ mov $((GDT_ENTRY_VGETCPU<<3)|3),%cx
+ lsl %ecx,%ecx
+ jnz 1f
+ push %ecx
+ and $0xff,%ecx /* 8-bit cpu number */
+ mov %ecx,(%eax)
+ pop %ecx
+ xor %eax,%eax
+ shr $8,%ecx /* assume top 4 bits are zero */
+ mov %ecx,(%edx)
+ ret
+1:
+ push $-EFAULT /* saves 2 bytes of .text */
+ pop %eax
+ ret
+.LEND_vgetcpu:
+ .size __vgetcpu,.-.LSTART_vgetcpu
+ .previous
+
+/* ZZZ: need CFI annotations here */
--- 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

/*
--- 2.6.17-32.orig/arch/i386/kernel/vsyscall-sigreturn.S
+++ 2.6.17-32/arch/i386/kernel/vsyscall-sigreturn.S
@@ -26,7 +26,7 @@ __kernel_sigreturn:
.LEND_sigreturn:
.size __kernel_sigreturn,.-.LSTART_sigreturn

- .balign 32
+ .balign 16
.globl __kernel_rt_sigreturn
.type __kernel_rt_sigreturn,@function
__kernel_rt_sigreturn:
@@ -35,7 +35,6 @@ __kernel_rt_sigreturn:
int $0x80
.LEND_rt_sigreturn:
.size __kernel_rt_sigreturn,.-.LSTART_rt_sigreturn
- .balign 32
.previous

.section .eh_frame,"a",@progbits
--
Chuck
"You can't read a newspaper if you can't read." --George W. Bush
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