how to set control register 0 (cr0) bits in x86-64 using gcc assembly on linux

Question

I am using the following code to set the cr0 bit to disable cache. When I compile this

#include <stdio.h>

int main()
{
        __asm__("pushl  %eax\n\t"
                "mov    %cr0,%eax;\n\t"
                "orl    $(1 << 30),%eax;\n\t"
                "mov    %eax,%cr0;\n\t"
                "wbinvd\n\t"
                "popl   %eax"
);

        return 0;
}

I am getting error saying that the operands are invalid for mov.

Can anyone please point me to a good gcc x86-64 guide for doing these kinds of things? Also what exactly is wrong with the above code?

Answer 1

Try this: "mov %%cr0, %%eax \n"

A simple % is interpreted as user argument (I think).

You should read this

Answer 2

The code compiles OK for me on 32-bit x86 bit not on x86-64 - this is with gcc 4.2.1 on Mac OS X:

$ gcc -Wall -m32 cr0.c -o cr0
$

No errors or warnings.

$ gcc -Wall -m64 cr0.c -o cr0
/var/folders/.../cce0FYAB.s:9:suffix or operands invalid for `push'
/var/folders/.../cce0FYAB.s:10:suffix or operands invalid for `mov'
/var/folders/.../cce0FYAB.s:12:suffix or operands invalid for `mov'
/var/folders/.../cce0FYAB.s:14:suffix or operands invalid for `pop'
$

So I guess there are deeper issues than just the mov %eax,%cr0 instruction here with asm on x86-64.

Looking at the x86-64 ISA it seems that you probably need something like this for x86-64:

#include <stdio.h>

int main()
{
        __asm__("pushq  %rax\n\t"
                "movq    %cr0,%rax\n\t"
                "orl    $(1 << 30),%eax\n\t"
                "movq    %rax,%cr0\n\t"
                "wbinvd\n\t"
                "popq   %rax"
);

        return 0;
}

I don't know if this works but it at least compiles/assembles OK:

$ gcc -Wall -m64 cr0.c -o cr0
$ 

Answer 3

You cannot do operations like this from user code and even running as root is user code.

You will need to make this into a driver module and load it using insmod.

Answer 4

Ok, so finally I wrote the following kernel module. Am not sure it is right, since I don't observe the drastic slowdown which should accompany when you disable cache. But this compiles and inserts properly.

Any pointers will be helpful.

Thanks!

#include <linux/init.h>
#include <linux/module.h>
MODULE_LICENSE("Dual BSD/GPL");
static int hello_init(void)
{
        printk(KERN_ALERT "Hello, world\n");
        __asm__("push   %rax\n\t"
                "mov    %cr0,%rax;\n\t"
                "or     $(1 << 30),%rax;\n\t"
                "mov    %rax,%cr0;\n\t"
                "wbinvd\n\t"
                "pop    %rax"
);
        return 0;
}
static void hello_exit(void)
{
        printk(KERN_ALERT "Goodbye, cruel world\n");
        __asm__("push   %rax\n\t"
                "mov    %cr0,%rax;\n\t"
                "and     $~(1 << 30),%rax;\n\t"
                "mov    %rax,%cr0;\n\t"
                "wbinvd\n\t"
                "pop    %rax"
);
}
module_init(hello_init);
module_exit(hello_exit);