//-------------------------------------------------------------------
//	pgfaults.c
//
//	This module installs a kernel function which intercepts all
//	page-fault exceptions in order to record statistics that an 
//	application program would be able to access (by reading the 
//	pseudo-file named '/proc/pgfaults'). 
//
//	programmer: ALLAN CRUSE
//	written on: 14 OCT 2004
//-------------------------------------------------------------------

#include <linux/module.h>	// for init_module() 
#include <linux/proc_fs.h>	// for create_proc_read_entry() 

#define PGF_ID	14

static char modname[] = "pgfaults";
static unsigned long reason[ 8 ];
static unsigned short oldidtr[3], newidtr[3];
static unsigned long long *oldidt, *newidt;
static unsigned long kpage, isr_orig, selector_cs;


static void pgf_intercept( unsigned long *tos )
{
	int	error_code = tos[ 10 ];

	reason[ error_code & 7 ] += 1;
}


//------  PAGE_FAULT EXCEPTION-HANDLER  --------//
asmlinkage void isr_page_fault( void );
asm("	.text					");
asm("	.type	isr_page_fault, @function	");
asm("isr_page_fault:				");
asm("	pushal					");
asm("	pushl	%ds				");
asm("	pushl	%es				");
//
asm("	movl	%ss, %eax			");
asm("	movl	%eax, %ds			");
asm("	movl	%eax, %es			");
//
asm("	pushl	%esp				");
asm("	call	pgf_intercept			");
asm("	addl	$4, %esp			");
//
asm("	popl	%es				");
asm("	popl	%ds				");
asm("	popal	   				");
asm("	jmp	*isr_orig			");
//----------------------------------------------//






static int my_get_info( char *buf, char **start, off_t off, int count )
{
	int	i, len;

	len = 0;
	len += sprintf( buf+len, "\nPage-Faults Summary\n" );
	len += sprintf( buf+len, "\nReason  Occurrences\n" );
	for (i = 0; i < 8; i++)
		{
		len += sprintf( buf+len, " %d%d%d:", (i>>2)&1, (i>>1)&1, i&1 );
		len += sprintf( buf+len, " %8d ", reason[ i ] );
		len += sprintf( buf+len, "\n" );
		}
	len += sprintf( buf+len, "\n" );

	return	len;
}


MODULE_LICENSE("GPL"); 





static void load_IDTR( void *regimage )
{
	asm(" lidt %0 " : : "m" (*(unsigned short *)regimage) );
}



void cleanup_module( void )
{
	// delete the pseudo-file 
	remove_proc_entry( modname, NULL );

	// deactivate our altered Interrupt Descriptor Table
	smp_call_function( load_IDTR, oldidtr, 1, 1 );
	load_IDTR( oldidtr );

	// release the allocated page of kernel memory
	if ( kpage ) free_page( kpage );
	printk( "<1>Removing \'%s\' module\n", modname );
}







int init_module( void )
{
	unsigned long long	gate_desc;

	printk( "<1>\nInstalling \'%s\' module\n", modname );

	// allocate a page of kernel memory to hold our new IDT
	kpage = get_free_page( GFP_KERNEL );
	if ( !kpage ) return -ENOMEM;

	// copy the Interrupt Descriptor Table to this 'writable' page 
	asm(" sidt oldidtr \n sidt newidtr \n movw %cs, selector_cs ");
	memcpy( newidtr+1, &kpage, sizeof( kpage ) );
	oldidt = (unsigned long long*)(*(unsigned long*)(oldidtr+1)); 
	newidt = (unsigned long long*)(*(unsigned long*)(newidtr+1)); 
	memcpy( newidt, oldidt, 256 * sizeof( unsigned long long ) );

	// initialize our exception-handler's indirect jump address
	gate_desc = oldidt[ PGF_ID ];
	gate_desc &= 0xFFFF00000000FFFFLL;
	gate_desc |= ( gate_desc >> 32 );
	isr_orig = (unsigned long)gate_desc;

	// modify our copy of the Interrupt Descriptor Table 
	gate_desc = (unsigned long long)isr_page_fault;
	gate_desc &= 0x00000000FFFFFFFFLL;
	gate_desc |= ( gate_desc << 32 );
	gate_desc &= 0xFFFF00000000FFFFLL;
	gate_desc |= 0x00008E0000000000LL;
	gate_desc |= ( selector_cs << 16 );
	newidt[ PGF_ID ] = gate_desc;	

	// now activate the new Interrupt Descriptor Table
	load_IDTR( newidtr );
	smp_call_function( load_IDTR, newidtr, 1, 1 );	
	
	// create the pseudo-file that lets users see page-fault counts
	create_proc_info_entry( modname, 0, NULL, my_get_info );
	return	0;  //SUCCESS
}