Obtaining Main ID (MIDR) etc.

I want to do something like the x86 CPUID instruction on ARM, but I couldn't execute the instruction to get CPU information (MRC) in the user space of Raspbian, so I made a kernel module.

Device driver that executes MRC instructions

Rt is excluded from the operands of the ARM MRC instruction. 　　coproc, #opcode1, CRn, CRm, #opcode2 , Address space

`python`


+-----+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|...20|19|18|17|16|15|14|13|12|11|10| 9| 8| 7| 6| 5| 4| 3| 2| 1| 0|
+-----+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|0..01|  coproc   |opcode1 |    CRn    |    CRm    |opcode2 |     |
+-----+-----------+--------+-----------+-----------+--------+-----+

Make it readable in the character device file / dev / arm_mrc, which looks like.

Since the data is 4 bytes, the lower 2 bits are the byte positions in the data.

The address is specified by the lseek function. The range from the beginning is 0x100000 to 0x1FFFFF.

The data is obtained with the read function.

important point

The result of an MRC instruction executed within the kernel module The information is not always common to kernels / users.
Execution of undefined instructions in kernel module Depending on the area, undefined instructions may be executed and processing may be interrupted.

Linux kernel module

There are three files. "make insmod" will bring up "/ dev / arm_mrc". The environment is Raspbian buster, Kernel Version 4.19.

Source code for Raspbian

`arm_mrc.c`


#include <uapi/linux/fs.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/uaccess.h>
#include <linux/slab.h>
#include <linux/vmalloc.h>
#include <linux/init.h>
#include <linux/cdev.h>
#include <linux/fs.h>
#include <asm/cacheflush.h>

MODULE_LICENSE("Dual BSD/GPL");
MODULE_AUTHOR("ikiuo");
MODULE_DESCRIPTION("Invoke MRC instruction");
MODULE_VERSION("0.1.0");

#if defined(DEBUG)
#define debug(x)  printk x
#else
#define debug(x)
#endif

typedef struct class class_type;
typedef struct device device_type;
typedef struct cdev chrdev_type;
typedef struct inode inode_type;
typedef struct file file_type;
typedef struct file_operations file_handler;

typedef unsigned int (*mrc_func)(void);

static void *mrc_proc = NULL;
#define INS_MRC       0xee100010
#define INS_MOV_PC_LR 0xe1a0f00e /* MOV PC,LR */
#define INS_BX_LR     0xe12fff1e /* BX  LR    */

/* addr = [1:mode][4:cop][3:op1][4:CRn][4:CRm][3:op2][2:00] */
static const loff_t offset_max = (1 << (1+4+3+4+4+3+2));

static loff_t
device_llseek (file_type *file, loff_t offset, int mode)
{
  debug((KERN_INFO "lseek(%p, %Ld, %d): call\n", file, offset, mode));

  switch (mode)
    {
    default: return -EINVAL;
    case SEEK_SET: offset += file->f_pos; break;
    case SEEK_CUR: break;
    case SEEK_END: offset += offset_max; break;
    }
  if (offset < 0) return -EINVAL;
  if (offset > offset_max) return -EINVAL;
  file->f_pos = offset;

  debug((KERN_INFO "lseek(%p, %Ld, %d): end\n", file, offset, mode));

  return offset;
}

static ssize_t
device_read (file_type *file, char *buffer, size_t size, loff_t *offset)
{
  loff_t addr, rem, pos = *offset;
  ssize_t total = 0;
  size_t res_size;
  loff_t res_pos;

  unsigned int *ins = (unsigned int *) mrc_proc;
  unsigned int cop, CRn, op1, CRm, op2;
  loff_t mode;
  unsigned int status;

  if (pos < 0) return -EIO;
  if (pos >= offset_max) return -EIO;

  rem = offset_max - pos;
  if (size > rem) size = rem;

  while (size > 0)
    {
      addr = pos >> 2;

      op2 = (addr >>  0) &  7;
      CRm = (addr >>  3) & 15;
      CRn = (addr >>  7) & 15;
      op1 = (addr >> 11) &  7;
      cop = (addr >> 14) & 15;
      mode = (addr >> 18);

      debug((KERN_INFO "read(%p, %p, %d, %Ld)\n", file, buffer, size, *offset));

      status = 0;
      if (mode != 1)
          return -EIO;

      ins[0] = INS_MRC | (op1 << 21) | (CRn << 16) | (cop << 8) | (op2 << 5) | CRm;
      ins[1] = INS_MOV_PC_LR;
      flush_kernel_vmap_range (ins, 16);
      flush_icache_range ((unsigned long)ins,16);
      status = ((mrc_func)ins) ();

      res_pos = pos & 3;
      res_size = 4 - res_pos;
      if (res_size > size)
        res_size = size;

      __copy_to_user (buffer, ((char*)&status) + res_pos, res_size);

      buffer += res_size;
      size   -= res_size;
      pos    += res_size;
      total  += res_size;
    }
  *offset = pos;
  return total;
}

static int
device_open (inode_type *inode, file_type *file)
{
  return (try_module_get (THIS_MODULE) ? 0 : -EBUSY);
}

static int
device_release (inode_type *inode, file_type *file)
{
  module_put (THIS_MODULE);
  return 0;
}

static file_handler handler = {
  .llseek  = device_llseek,
  .read    = device_read,
  .open    = device_open,
  .release = device_release,
};
static chrdev_type cdev_mrc;

#define DEVICE_NAME "arm_mrc"
static const int arm_ins_buffer_type = 1;
static const char *device_name = DEVICE_NAME;
static int major_number = 0;
static int minor_base = 0;
static int minor_count = 1;
static class_type *device_class = NULL;
static device_type *sys_device = NULL;

static void *
arm_alloc_ins_buffer (void)
{
  if (arm_ins_buffer_type)
    return __vmalloc (256, GFP_KERNEL, PAGE_KERNEL_EXEC);
  else
    return kmalloc (256, GFP_KERNEL);
}

static void
arm_free_ins_buffer (void *p)
{
  if (p == NULL)
    return;
  if (arm_ins_buffer_type)
    vfree (p);
  else
    kfree (p);
}

static void
arm_mrc_exit_chrdev (void)
{
  dev_t dev = MKDEV (major_number, minor_base);
  unregister_chrdev_region (dev, minor_count);
  arm_free_ins_buffer (mrc_proc);
}

static void
arm_mrc_exit_cdev_chrdev (void)
{
  cdev_del (&cdev_mrc);
  arm_mrc_exit_chrdev ();
}

static void
arm_mrc_exit_all (void)
{
  device_destroy (device_class, MKDEV (major_number, minor_base));
  class_destroy (device_class);
  arm_mrc_exit_cdev_chrdev ();
}

static int
__init arm_mrc_init (void)
{
  dev_t dev;
  int stat;

  mrc_proc = arm_alloc_ins_buffer ();
  if (mrc_proc == NULL) return -ENOMEM;

  stat = alloc_chrdev_region (&dev, minor_base, minor_count, device_name);
  if (stat < 0)
    {
      printk (KERN_ERR DEVICE_NAME ": alloc_chrdev_region failed: %d\n", stat);
      return stat;
    }

  major_number = MAJOR (dev);
  cdev_init (&cdev_mrc, &handler);
  cdev_mrc.owner = THIS_MODULE;

  stat = cdev_add (&cdev_mrc, dev, minor_count);
  if (stat < 0)
    {
      printk (KERN_ERR DEVICE_NAME ": cdev_add failed: %d\n", stat);
      arm_mrc_exit_chrdev ();
      return stat;
    }

  device_class = class_create (THIS_MODULE, device_name);
  if (IS_ERR(device_class))
    {
      printk (KERN_ERR DEVICE_NAME ": class_create failed\n");
      arm_mrc_exit_cdev_chrdev ();
      return -1;
    }

  sys_device = device_create(device_class, NULL, dev, NULL, DEVICE_NAME);
  if (IS_ERR(sys_device))
    {
      arm_mrc_exit_all ();
      return -1;
    }

  return 0;
}

static void
__exit arm_mrc_exit (void)
{
  arm_mrc_exit_all ();
}

module_init (arm_mrc_init);
module_exit (arm_mrc_exit);

`Makefile`


# Makefile

#MOD_DIR = /lib/modules/4.9.0-6-rpi
#MOD_DIR = /lib/modules/4.19.93+
#MOD_DIR = /lib/modules/4.19.93-v7+
MOD_DIR = /lib/modules/4.19.93-v7l+

DEVNAME = arm_mrc
KMOD    = $(DEVNAME).ko

obj-m += $(DEVNAME).o

all:	build

build:	$(KMOD)

$(KMOD): $(DEVNAME).c
	make -C $(MOD_DIR)/build M=$(PWD) modules

clean:
	make -C $(MOD_DIR)/build M=$(PWD) clean

insmod: build
	sudo cp 99-arm_mrc.rules /etc/udev/rules.d/
	sudo insmod $(KMOD)

rmmod:
	sudo rmmod $(KMOD)

`99-arm_mrc.rules`


KERNEL=="arm_mrc", GROUP="root", MODE="0666"

Test program

`test.c`


#include <stdio.h>
#include <stdlib.h>

#include <errno.h>
#include <fcntl.h>
#include <unistd.h>

int
main (int argc, char **argv)
{
  int debug = 0;
  int coproc = 0;
  int opcode1 = 0;
  int CRn = 0;
  int CRm = 0;
  int opcode2 = 0;

  int fd;
  off_t addr, spos;
  ssize_t rdsz;

  unsigned int value;
  int exitcode = 0;

  if (argc < 2)
    {
      printf ("Usage: %s coproc [opcode1 [CRn [CRm [opcode2]]]]\n", argv[0]);
      return 1;
    }

  if (argc > 1) coproc  = strtol (argv[1], NULL, 10);
  if (argc > 2) opcode1 = strtol (argv[2], NULL, 10);
  if (argc > 3) CRn     = strtol (argv[3], NULL, 10);
  if (argc > 4) CRm     = strtol (argv[4], NULL, 10);
  if (argc > 5) opcode2 = strtol (argv[5], NULL, 10);

  if ((coproc > 15) || (opcode1 > 7) || (CRn > 15)
      || (CRm > 15) || (opcode2 > 7))
    {
      printf ("invalid argument.\n");
      printf ("coproc=%d, opcode1=%d, CRn=%d, CRm=%d, opcode2=%d\n",
              coproc, opcode1, CRn, CRm, opcode2);
      return 1;
    }
  addr = ((1 << 20) | (coproc << 16) | (opcode1 << 13)
          | (CRn << 9) | (CRm << 5) | (opcode2 << 2));

  fd = open ("/dev/arm_mrc", O_RDONLY);
  if (fd < 0)
    {
      printf ("Can't open '/dev/arm_mrc': error=%d (%d)\n", fd, errno);
      return 2;
    }

  spos = lseek (fd, addr, SEEK_SET);
  if (addr != spos)
    {
      printf ("seek error: error=%d (%d)\n", spos, errno);
      exitcode = 2;
    }
  else
    {
      rdsz = read (fd, &value, sizeof (value));
      if (rdsz != sizeof (value))
        {
          printf ("read error: error=%d (%d)\n", spos, errno);
          exitcode = 2;
        }
      else
        {
          printf ("MRC p%d,#%d,Rt,c%d,c%d,#%d; Rt=%#010x\n",
                  coproc, opcode1, CRn, CRm, opcode2, value);
        }
    }

  close (fd);
  return 0;
}

Execution result of test program on RaspberryPi 4

`MIDR_EL1`


$ ./test 15 0 0 0 0
MRC p15,0,Rt,c0,c0,0; Rt=0x410fd083

`ID_PFR0_EL1`


./test 15 0 0 1 0
MRC p15,0,Rt,c0,c1,0; Rt=0x00000131

`ID_PFR1_EL1`


$ ./test 15 0 0 1 1
MRC p15,0,Rt,c0,c1,1; Rt=0x00011011

Device driver to get CPU information for ARM

Obtaining Main ID (MIDR) etc.

Device driver that executes MRC instructions

python

important point

Linux kernel module

arm_mrc.c

Makefile

99-arm_mrc.rules

test.c

MIDR_EL1

ID_PFR0_EL1

ID_PFR1_EL1