LinuxQuestions.org - [SOLVED] Disabling CPU caches

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Disabling CPU caches

Hi,

I would like to disable the level 1 and level 2 caches of my CPU. I wrote a kernel module to set the 30th bit of the control register cr0. But when I try to insert the module with insmod, the system freezes. I am using a Thinkpad X41 with a Pentium M and kernel 2.6.32-40-generic #87-Ubuntu SMP. What can I do to determine the reason for the freeze?

Thank you for your help!

The code of the module:

Code:

#include <linux/init.h>

#include <linux/module.h>

MODULE_LICENSE("Dual BSD/GPL");

static int disableCache_init(void)

{

        printk(KERN_ALERT "Disabling L1 and L2 caches.\n");

        __asm__(".intel_syntax noprefix\n\t"

                "mov    eax,cr0\n\t"

                "or    eax,(1 << 30)\n\t"

                "mov    cr0,eax\n\t"

                "wbinvd\n\t"

                ".att_syntax noprefix\n\t"

        : : : "eax" );

        return 0;

}

static void disableCache_exit(void)

{

        printk(KERN_ALERT "Enabling L1 and L2 caches.\n");

        __asm__(".intel_syntax noprefix\n\t"

                "mov    eax,cr0\n\t"

                "and    eax,~(1 << 30)\n\t"

                "mov    cr0,eax\n\t"

                "wbinvd\n\t"

                ".att_syntax noprefix\n\t"

        : : : "eax" );

}

module_init(disableCache_init);

module_exit(disableCache_exit);

And the Makefile:

Code:

EXTRA_CFLAGS = -m32

obj-m += disableCache.o



all:

        make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

insmod command:

Code:

sudo insmod ./disableCache.ko

Edit: Adopted the code of the module, see second post.

As I just found out, the syntax for the mov instruction with control registers is not as I expected: in Intel syntax, the destination register comes first in cases where control registers are involved. I adopted the code of the module in the first post.

However, the overall result stays the same, the system freezes when the module is inserted. I found out that the module runs without problems with this line commented out:

Code:

"or eax,(1 << 30)\n\t"

(But it also has no effect because cr0 is not altered.)

Any ideas?

Any suggestions about an other forum where I can ask? Any hint is appreciated, thanks.

You can't disable CPU cache in fly.

Disabling cpu caches gives you a pentium I like processor!!!
The addition of Cache on CPU was one of the best upgrades to cpu architecture and is used to retain temporal data / instructions. as nini09 have said, you cannot disable it at fly, but certain BIOS would give you the option at boot time. Is there any special requirement, or is only homework / test?

It can be done this is from memtest

Code:

static inline void cache_off(void)

{

        asm(

                "push %eax\n\t"

                "movl %cr0,%eax\n\t"

                "orl $0x40000000,%eax\n\t"  /* Set CD */

                "movl %eax,%cr0\n\t"

                "wbinvd\n\t"

                "pop  %eax\n\t");

}

but you need to flush the cache that's what wbinvd is for.

Thank you for your answers. I want to disable the caches for a research project where we want to measure execution times and want to be sure they are not influenced by the internal caches.

@whizje: I tried your suggestion and apparently, it is working. :) But I have to investigate it further tomorrow when I will have more time.

After some investigation, I found out that the module runs fine (with the corrections from my second post). I just didn't wait long enough after fixing the mov cr0, eax bug, misinterpreating the slow behavior of the system with disabled caches as a complete freeze. My apologies for that.

The code posted by whizje is also working, of course. I didn't find an option for cache disabling in the BIOS of my machine.

Thanks for your input.

how to run the above segment of codes....thanks

i am trying to disable the caches ...i want to know how the above code works and how to execute it

@hga Run make and insmod. Source code, makefile and argument for insmod is given in the first post. If you have problems, please ask more specific questions.

can you be more specfic with the steps...where is to be the code saved and with what extension? thank you.

I would suggest to save the source code (in an arbitrary directory) in a file named

Code:

disableCache.c

and the make file in a file named

Code:

Makefile

Then run

Code:

make all

followed by

Code:

sudo insmod ./disableCache.ko

i saved the above files and tried the make command. But it displays
make: Nothing to be done for `all'..... Can you help me???

---------- Post added 07-31-12 at 07:42 AM ----------

i saved the above files and tried the make command. But it displays
make: Nothing to be done for `all'..... Can you help me???

I have no idea, I have only a very basic knowledge of make...

Hi naam,

It look like your makefile has problem. You can make Hello module work at first and then deal with disableCache.c