Based on the Wikipedia entry as well as the Intel manual, rdpmc
should be available to user-mode processes as long as bit 8
of CR4
is set. However, I am still running into general protection
error when trying to run rdpmc
from userspace even with that bit set.
I am running on an 8-core Intel X3470
on kernel 2.6.32-279.el6.x86_64
.
Here is the user-mode program I am trying to execute:
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <inttypes.h>
#include <sched.h>
#include <assert.h>
uint64_t
read_pmc(int ecx)
{
unsigned int a, d;
__asm __volatile("rdpmc" : "=a"(a), "=d"(d) : "c"(ecx));
return ((uint64_t)a) | (((uint64_t)d) << 32);
}
int main(int ac, char **av)
{
uint64_t start, end;
cpu_set_t cpuset;
unsigned int c;
int i;
if (ac != 3) {
fprintf(stderr, "usage: %s cpu-id pmc-num\n", av[0]);
exit(EXIT_FAILURE);
}
i = atoi(av[1]);
c = atoi(av[2]);
CPU_ZERO(&cpuset);
CPU_SET(i, &cpuset);
assert(sched_setaffinity(0, sizeof(cpuset), &cpuset) == 0);
printf("%lu\n", read_pmc(c));
return 0;
}
Here is the kernel module which sets the bit and reads out CR4
so I can manually verify that the bit has been set.
/*
* Enable PMC in user mode.
*/
#include <linux/module.h>
#include <linux/kernel.h>
int init_module(void)
{
typedef long unsigned int uint64_t;
uint64_t output;
// Set CR4, Bit 8 to enable PMC
__asm__("push %rax\n\t"
"mov %cr4,%rax;\n\t"
"or $(1 << 7),%rax;\n\t"
"mov %rax,%cr4;\n\t"
"wbinvd\n\t"
"pop %rax"
);
// Read back CR4 to check the bit.
__asm__("\t mov %%cr4,%0" : "=r"(output));
printk(KERN_INFO "%lu", output);
return 0;
}
void cleanup_module(void)
{
__asm__("push %rax\n\t"
"push %rbx\n\t"
"mov %cr4,%rax;\n\t"
"mov $(1 << 7), %rbx\n\t"
"not %rbx\n\t"
"and %rbx, %rax;\n\t"
"mov %rax,%cr4;\n\t"
"wbinvd\n\t"
"pop %rbx\n\t"
"pop %rax\n\t"
);
}
Apparently, when Intel says Bit 8
, they are referring to the 9th bit from the right, since their indexing begins at 0
. Replacing $(1 << 7)
with $(1 << 8)
globally resolves the issue, and allows rdpmc
to be called from user mode.
Here is the updated kernel module, also using on_each_cpu
to make sure that it is set on every core.
/*
* Read PMC in kernel mode.
*/
#include <linux/module.h> /* Needed by all modules */
#include <linux/kernel.h> /* Needed for KERN_INFO */
static void printc4(void) {
typedef long unsigned int uint64_t;
uint64_t output;
// Read back CR4 to check the bit.
__asm__("\t mov %%cr4,%0" : "=r"(output));
printk(KERN_INFO "%lu", output);
}
static void setc4b8(void * info) {
// Set CR4, Bit 8 (9th bit from the right) to enable
__asm__("push %rax\n\t"
"mov %cr4,%rax;\n\t"
"or $(1 << 8),%rax;\n\t"
"mov %rax,%cr4;\n\t"
"wbinvd\n\t"
"pop %rax"
);
// Check which CPU we are on:
printk(KERN_INFO "Ran on Processor %d", smp_processor_id());
printc4();
}
static void clearc4b8(void * info) {
printc4();
__asm__("push %rax\n\t"
"push %rbx\n\t"
"mov %cr4,%rax;\n\t"
"mov $(1 << 8), %rbx\n\t"
"not %rbx\n\t"
"and %rbx, %rax;\n\t"
"mov %rax,%cr4;\n\t"
"wbinvd\n\t"
"pop %rbx\n\t"
"pop %rax\n\t"
);
printk(KERN_INFO "Ran on Processor %d", smp_processor_id());
}
int init_module(void)
{
on_each_cpu(setc4b8, NULL, 0);
return 0;
}
void cleanup_module(void)
{
on_each_cpu(clearc4b8, NULL, 0);
}