Use fast in-situ test for gate A20 being set, to cut down on the

number of (potentially very slow) gateA20_set operations.

Die with a fatal error if we are unable to set gate A20; if this fails
then we are bound to experience memory corruption at a later stage,
and I'd prefer to pick it up early.

diff --git a/src/arch/i386/firmware/pcbios/gateA20.c b/src/arch/i386/firmware/pcbios/gateA20.c
index cdc8668..c424f12 100644 (file)
--- a/src/arch/i386/firmware/pcbios/gateA20.c
+++ b/src/arch/i386/firmware/pcbios/gateA20.c
@@ -1,3 +1,4 @@
+#include <stdio.h>
 #include "realmode.h"
 #include "timer.h"
 #include "latch.h"
@@ -41,6 +42,35 @@ static void empty_8042 ( void ) {
 }
 #endif /* IBM_L40 */
 
+/**
+ * Fast test to see if gate A20 is already set
+ *
+ * @ret set            Gate A20 is set
+ */
+static int gateA20_is_set ( void ) {
+       static uint32_t test_pattern = 0xdeadbeef;
+       physaddr_t test_pattern_phys = virt_to_phys ( &test_pattern );
+       physaddr_t verify_pattern_phys = ( test_pattern_phys ^ 0x100000 );
+       userptr_t verify_pattern_user = phys_to_user ( verify_pattern_phys );
+       uint32_t verify_pattern;
+
+       /* Check for difference */
+       copy_from_user ( &verify_pattern, verify_pattern_user, 0,
+                        sizeof ( verify_pattern ) );
+       if ( verify_pattern != test_pattern )
+               return 1;
+
+       /* Invert pattern and retest, just to be sure */
+       test_pattern ^= 0xffffffff;
+       copy_from_user ( &verify_pattern, verify_pattern_user, 0,
+                        sizeof ( verify_pattern ) );
+       if ( verify_pattern != test_pattern )
+               return 1;
+
+       /* Pattern matched both times; gate A20 is not set */
+       return 0;
+}
+
 /*
  * Gate A20 for high memory
  *
@@ -52,35 +82,41 @@ static void empty_8042 ( void ) {
 void gateA20_set ( void ) {
        static char reentry_guard = 0;
        unsigned int discard_a;
-       unsigned int flags;
 
+       /* Avoid potential infinite recursion */
        if ( reentry_guard )
                return;
        reentry_guard = 1;
 
-       __asm__ __volatile__ ( REAL_CODE ( "sti\n\t"
-                                          "stc\n\t"
-                                          "int $0x15\n\t"
-                                          "pushfw\n\t"
-                                          "popw %w0\n\t"
-                                          "cli\n\t" )
-                              : "=r" ( flags ), "=a" ( discard_a )
-                              : "a" ( Enable_A20 ) );
-
+       /* Fast check to see if gate A20 is already enabled */
+       if ( gateA20_is_set() )
+               goto out;
 
-       if ( flags & CF ) {
-               /* INT 15 method failed, try alternatives */
+       /* Try INT 15 method first */
+       __asm__ __volatile__ ( REAL_CODE ( "int $0x15" )
+                              : "=a" ( discard_a )
+                              : "a" ( Enable_A20 ) );
+       if ( gateA20_is_set() )
+               goto out;
+       
+       /* INT 15 method failed, try alternatives */
 #ifdef IBM_L40
-               outb(0x2, 0x92);
+       outb(0x2, 0x92);
 #else  /* IBM_L40 */
-               empty_8042();
-               outb(KC_CMD_WOUT, K_CMD);
-               empty_8042();
-               outb(KB_SET_A20, K_RDWR);
-               empty_8042();
+       empty_8042();
+       outb(KC_CMD_WOUT, K_CMD);
+       empty_8042();
+       outb(KB_SET_A20, K_RDWR);
+       empty_8042();
 #endif /* IBM_L40 */
-       }
+       if ( gateA20_is_set() )
+               goto out;
+
+       /* Better to die now than corrupt memory later */
+       printf ( "FATAL: Gate A20 stuck\n" );
+       while ( 1 ) {}
 
+ out:
        reentry_guard = 0;
 }