linux-kernel - Disable L1/L2/L3 cache and MTRR

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Message-ID: <AANLkTikKFnVOFr70u_2Hdtz0mXuAEckWWUy885m_ut--@mail.gmail.com>
Date:	Thu, 21 Oct 2010 12:21:15 -0400
From:	Andev <debiandev@...il.com>
To:	linux kernel <kernelnewbies@...linux.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Disable L1/L2/L3 cache and MTRR

Hello,

You need to set the 30th bit of CR0 register to disable the cache.
I've tried disabling the L1/L2/L3 cache in an intel processor as follows.

#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"
                "pop    %rax"
);
}
module_init(hello_init);
module_exit(hello_exit);

When I try to remove this module, the system hangs! Any help?

In Intel software developers manual, it is mentioned that apart from the
above you need to disable MTRR. I did that using the following command:

echo "disable=00" >| /proc/mtrr

Now when I run some sample benchmarks they show a slowdown of almost 1000x!!

This is not reasonable since the max. The slowdown I was expecting is 200x
considering that it will take 200 cycles to read from DRAM.

Thanks,
Andev.
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