[rng] Check for functioning RTC interrupt

On some platforms (observed in a small subset of Microsoft Azure
(Hyper-V) virtual machines), the RTC appears to be incapable of
generating an interrupt via the legacy PIC.  The RTC status registers
show that a periodic interrupt has been asserted, but the PIC IRR
shows that IRQ8 remains inactive.

On such systems, iPXE will currently freeze during the "iPXE
initialising devices..." message.

Work around this problem by checking that RTC interrupts are being
raised before returning from rtc_entropy_enable().  If no interrupt is
seen within 100ms, then we assume that the RTC interrupt mechanism is
broken.  In these circumstances, iPXE will continue to initialise but
any subsequent attempt to generate entropy will fail.  In particular,
HTTPS connections will fail with an error indicating that no entropy
is available.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/x86/include/bits/errfile.h

 #define ERRFILE_vesafb		( ERRFILE_ARCH | ERRFILE_CORE | 0x000c0000 )
 #define ERRFILE_int13con	( ERRFILE_ARCH | ERRFILE_CORE | 0x000d0000 )
 #define ERRFILE_gdbmach		( ERRFILE_ARCH | ERRFILE_CORE | 0x000e0000 )
+#define ERRFILE_rtc_entropy	( ERRFILE_ARCH | ERRFILE_CORE | 0x000f0000 )
 
 #define ERRFILE_bootsector     ( ERRFILE_ARCH | ERRFILE_IMAGE | 0x00000000 )
 #define ERRFILE_bzimage	       ( ERRFILE_ARCH | ERRFILE_IMAGE | 0x00010000 )

src/arch/x86/interface/pcbios/rtc_entropy.c

 
 #include <stdint.h>
 #include <string.h>
+#include <errno.h>
+#include <unistd.h>
 #include <biosint.h>
 #include <pic8259.h>
 #include <rtc.h>
 #include <ipxe/entropy.h>
 
+/** Maximum time to wait for an RTC interrupt, in milliseconds */
+#define RTC_MAX_WAIT_MS 100
+
 /** RTC interrupt handler */
 extern void rtc_isr ( void );
 
 /** Previous RTC interrupt handler */
 static struct segoff rtc_old_handler;
 
+/** Flag set by RTC interrupt handler */
+extern volatile uint8_t __text16 ( rtc_flag );
+#define rtc_flag __use_text16 ( rtc_flag )
+
 /**
  * Hook RTC interrupt handler
  *
 static void rtc_enable_int ( void ) {
 	uint8_t status_b;
 
+	/* Clear any stale pending interrupts via status register C */
+	outb ( ( RTC_STATUS_C | CMOS_DISABLE_NMI ), CMOS_ADDRESS );
+	inb ( CMOS_DATA );
+
 	/* Set Periodic Interrupt Enable bit in status register B */
 	outb ( ( RTC_STATUS_B | CMOS_DISABLE_NMI ), CMOS_ADDRESS );
 	status_b = inb ( CMOS_DATA );
 	inb ( CMOS_DATA ); /* Discard; may be needed on some platforms */
 }
 
+/**
+ * Check that entropy gathering is functional
+ *
+ * @ret rc		Return status code
+ */
+static int rtc_entropy_check ( void ) {
+	unsigned int i;
+
+	/* Check that RTC interrupts are working */
+	rtc_flag = 0;
+	for ( i = 0 ; i < RTC_MAX_WAIT_MS ; i++ ) {
+
+		/* Allow interrupts to occur */
+		__asm__ __volatile__ ( "sti\n\t"
+				       "nop\n\t"
+				       "nop\n\t"
+				       "cli\n\t" );
+
+		/* Check for RTC interrupt flag */
+		if ( rtc_flag )
+			return 0;
+
+		/* Delay */
+		mdelay ( 1 );
+	}
+
+	DBGC ( &rtc_flag, "RTC timed out waiting for interrupt\n" );
+	return -ETIMEDOUT;
+}
+
 /**
  * Enable entropy gathering
  *
  * @ret rc		Return status code
  */
 static int rtc_entropy_enable ( void ) {
+	int rc;
 
+	/* Hook ISR and enable RTC interrupts */
 	rtc_hook_isr();
 	enable_irq ( RTC_IRQ );
 	rtc_enable_int();
 
+	/* Check that RTC interrupts are working */
+	if ( ( rc = rtc_entropy_check() ) != 0 )
+		goto err_check;
+
 	return 0;
+
+ err_check:
+	rtc_disable_int();
+	disable_irq ( RTC_IRQ );
+	rtc_unhook_isr();
+	return rc;
 }
 
 /**
  */
 static void rtc_entropy_disable ( void ) {
 
+	/* Disable RTC interrupts and unhook ISR */
 	rtc_disable_int();
 	disable_irq ( RTC_IRQ );
 	rtc_unhook_isr();