ipxe/src/arch/i386/interface/pcbios/rtc_entropy.c

/*
 * Copyright (C) 2012 Michael Brown <mbrown@fensystems.co.uk>.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation; either version 2 of the
 * License, or any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
 * 02110-1301, USA.
 */

FILE_LICENCE ( GPL2_OR_LATER );

/** @file
 *
 * RTC-based entropy source
 *
 */

#include <stdint.h>
#include <string.h>
#include <biosint.h>
#include <pic8259.h>
#include <rtc.h>
#include <ipxe/entropy.h>

/** RTC "interrupt triggered" flag */
static uint8_t __text16 ( rtc_flag );
#define rtc_flag __use_text16 ( rtc_flag )

/** RTC interrupt handler */
extern void rtc_isr ( void );

/** Previous RTC interrupt handler */
static struct segoff rtc_old_handler;

/**
 * Hook RTC interrupt handler
 *
 */
static void rtc_hook_isr ( void ) {

	/* RTC interrupt handler */
	__asm__ __volatile__ (
		TEXT16_CODE ( "\nrtc_isr:\n\t"
			      /* Preserve registers */
			      "pushw %%ax\n\t"
			      /* Set "interrupt triggered" flag */
			      "cs movb $0x01, %c0\n\t"
			      /* Read RTC status register C to
			       * acknowledge interrupt
			       */
			      "movb %3, %%al\n\t"
			      "outb %%al, %1\n\t"
			      "inb %2\n\t"
			      /* Send EOI */
			      "movb $0x20, %%al\n\t"
			      "outb %%al, $0xa0\n\t"
			      "outb %%al, $0x20\n\t"
			      /* Restore registers and return */
			      "popw %%ax\n\t"
			      "iret\n\t" )
		:
		: "p" ( __from_text16 ( &rtc_flag ) ),
		  "i" ( CMOS_ADDRESS ), "i" ( CMOS_DATA ),
		  "i" ( RTC_STATUS_C ) );

	hook_bios_interrupt ( RTC_INT, ( unsigned int ) rtc_isr,
			      &rtc_old_handler );
}

/**
 * Unhook RTC interrupt handler
 *
 */
static void rtc_unhook_isr ( void ) {
	int rc;

	rc = unhook_bios_interrupt ( RTC_INT, ( unsigned int ) rtc_isr,
				     &rtc_old_handler );
	assert ( rc == 0 ); /* Should always be able to unhook */
}

/**
 * Enable RTC interrupts
 *
 */
static void rtc_enable_int ( void ) {
	uint8_t status_b;

	/* Set Periodic Interrupt Enable bit in status register B */
	outb ( ( RTC_STATUS_B | CMOS_DISABLE_NMI ), CMOS_ADDRESS );
	status_b = inb ( CMOS_DATA );
	outb ( ( RTC_STATUS_B | CMOS_DISABLE_NMI ), CMOS_ADDRESS );
	outb ( ( status_b | RTC_STATUS_B_PIE ), CMOS_DATA );

	/* Re-enable NMI and reset to default address */
	outb ( CMOS_DEFAULT_ADDRESS, CMOS_ADDRESS );
	inb ( CMOS_DATA ); /* Discard; may be needed on some platforms */
}

/**
 * Disable RTC interrupts
 *
 */
static void rtc_disable_int ( void ) {
	uint8_t status_b;

	/* Clear Periodic Interrupt Enable bit in status register B */
	outb ( ( RTC_STATUS_B | CMOS_DISABLE_NMI ), CMOS_ADDRESS );
	status_b = inb ( CMOS_DATA );
	outb ( ( RTC_STATUS_B | CMOS_DISABLE_NMI ), CMOS_ADDRESS );
	outb ( ( status_b & ~RTC_STATUS_B_PIE ), CMOS_DATA );

	/* Re-enable NMI and reset to default address */
	outb ( CMOS_DEFAULT_ADDRESS, CMOS_ADDRESS );
	inb ( CMOS_DATA ); /* Discard; may be needed on some platforms */
}

/**
 * Enable entropy gathering
 *
 * @ret rc		Return status code
 */
static int rtc_entropy_enable ( void ) {

	rtc_hook_isr();
	enable_irq ( RTC_IRQ );
	rtc_enable_int();

	return 0;
}

/**
 * Disable entropy gathering
 *
 */
static void rtc_entropy_disable ( void ) {

	rtc_disable_int();
	disable_irq ( RTC_IRQ );
	rtc_unhook_isr();
}

/**
 * Measure a single RTC tick
 *
 * @ret delta		Length of RTC tick (in TSC units)
 */
uint8_t rtc_sample ( void ) {
	uint32_t before;
	uint32_t after;
	uint32_t temp;

	__asm__ __volatile__ (
		REAL_CODE ( /* Enable interrupts */
			    "sti\n\t"
			    /* Wait for RTC interrupt */
			    "cs movb %b2, %c4\n\t"
			    "\n1:\n\t"
			    "cs xchgb %b2, %c4\n\t" /* Serialize */
			    "testb %b2, %b2\n\t"
			    "jz 1b\n\t"
			    /* Read "before" TSC */
			    "rdtsc\n\t"
			    /* Store "before" TSC on stack */
			    "pushl %0\n\t"
			    /* Wait for another RTC interrupt */
			    "xorb %b2, %b2\n\t"
			    "cs movb %b2, %c4\n\t"
			    "\n1:\n\t"
			    "cs xchgb %b2, %c4\n\t" /* Serialize */
			    "testb %b2, %b2\n\t"
			    "jz 1b\n\t"
			    /* Read "after" TSC */
			    "rdtsc\n\t"
			    /* Retrieve "before" TSC on stack */
			    "popl %1\n\t"
			    /* Disable interrupts */
			    "cli\n\t"
			    )
		: "=a" ( after ), "=d" ( before ), "=q" ( temp )
		: "2" ( 0 ), "p" ( __from_text16 ( &rtc_flag ) ) );

	return ( after - before );
}

PROVIDE_ENTROPY_INLINE ( rtc, min_entropy_per_sample );
PROVIDE_ENTROPY ( rtc, entropy_enable, rtc_entropy_enable );
PROVIDE_ENTROPY ( rtc, entropy_disable, rtc_entropy_disable );
PROVIDE_ENTROPY_INLINE ( rtc, get_noise );