Read MSRs by accessing the Linux-provided CPU MSR device interface

preadmsr.c

/**
 * Copyright 2022 Roi L.
 * SPDX-License-Identifier: GPL-3.0-only
 * 
 * Read Machine-Specific-Register values by accessing the Linux-provided CPU MSR interface.
 */

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <assert.h>
#include <stdlib.h>
#include <inttypes.h>

enum Bool { FALSE, TRUE }; /* Just cause */

#define MAX_F_SIZE	            (20)
#define CPU_ID_MSR              (1u << (5))

#define DIE(msg) do { \
    printf("%s\n", msg); exit(-1); \
} while (0)

#define ASSERT(expr)    assert(expr)

#define CRAFT_PATH(buffer, cpun) \
    snprintf(buffer, MAX_F_SIZE, "/dev/cpu/%d/msr", cpun);

static char f[MAX_F_SIZE]; /* Global MSR path, replaced on each call if needed */

static uint64_t pread_msr(off_t reg_offset, int cpun)
{
    int msfd;
    uint64_t data = 0;

    CRAFT_PATH(f, cpun);
    if ((msfd = open(f, O_RDONLY)) == -1) 
        DIE("Failed to open MSR file.");
    if (pread(msfd, &data, sizeof(data), reg_offset) < 0)
        DIE("Failed to pread MSR value.");
    close(msfd);

    return data;
}

/* Make sure our CPU has MSR support via cpuid */
static inline int CPU_MSR_SUPPORT(void)
{
    uint32_t edx, eax = 1;
    asm volatile(".byte 0x0f, 0xa2" : "=d"(edx) : "0"(eax));  
    return (edx & CPU_ID_MSR) ? TRUE : FALSE;
}

int main(int argc, char const *argv[])
{
    ASSERT(CPU_MSR_SUPPORT() == TRUE);
    if (argc < 3) DIE("Usage: ./main <CPU NUMBER> <MSR_ADDRESS>");

    uint64_t data;
    
    data = pread_msr(strtol(argv[2], NULL, 16), atoi(argv[1]));
	if (data)
        printf("%" PRIx64 "\n", data);
	return 0;
}