reading reg from kernel driver module

[Ravindran Shanmugam]

Hi,

How to read "/dev/cpu/0/msr" from a kernel driver. I think  msr is a model specific register in intel.  Datasheet says 'msr' can be accessed with rdmsr instruction and I need to read msr offset 0x1A2 and 0x19C (to read the core temperature which is available in msr#1a2). Not sure, how to invoke the assembly instruction in the driver code.

Any idea.?

Regards,

Ravi

[Anil Kumar Pugalia]

Typically, for such things there should be some kernel function or macro mapped to the assembly code. Search for something in the arch specific headers related to core (temperature) etc

Thanks & Regards
Anil

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Ravindran Shanmugam wrote on 21/10/20 11:14 pm:

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[muralidhara mk]

Hi Ravindran,

You can use this simple example to read the MSR register addresses 0x1A2 and 0x19C

#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <stdint.h>
#include <stdlib.h>
#include <unistd.h>
#include <math.h>

void rdmsr_on_cpu(uint64_t, int);
void read_msr_reg(int, uint64_t, int);

int main(int argc, char *argv[])
{
//      uint64_t reg = 0x1A2;
        uint64_t reg = 0x19C;
        int cpu = 0;   // Pass required CPU number

        rdmsr_on_cpu(reg, cpu);
}

void read_msr_reg(int fd, uint64_t reg, int cpu)
{
        uint64_t data;

        if (pread(fd, &data, sizeof(data), reg) != sizeof(data)) {
                printf("##### pread error\n");
                if (errno == EIO) {
                        fprintf(stderr, "rdmsr: CPU %d cannot read "
                                "MSR 0x%lx\n", cpu, reg);
                        exit(4);
                } else {
                        perror("rdmsr: pread");
                        exit(127);
                }
        }
//      data = (data & 0x1f00) >> 8;  Read particular bits as per spec
        printf("MSR reg data: %lx\n", data);
}

void rdmsr_on_cpu(uint64_t reg, int cpu)
{
        int i, fd;
        char msr_file_name[64];

        sprintf(msr_file_name, "/dev/cpu/%d/msr", cpu);

        fd = open(msr_file_name, O_RDWR);
        if (fd < 0) {
                if (errno == ENXIO) {
                        fprintf(stderr, "rdmsr: No CPU %d\n", cpu);
                        exit(2);
                } else if (errno == EIO) {
                        fprintf(stderr, "rdmsr: CPU %d doesn't support MSRs\n", cpu);
                        exit(3);
                } else {
                        perror("rdmsr: open failed");
                        exit(127);
                }
        }

        read_msr_reg(fd, reg, cpu);

        close(fd);
}

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[Ravindran Shanmugam]

Thanks Muaralidhara, Anil for the inputs. Missed this for a while, let me try this solution.

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