[prefix] When we have to hook INT 19, chain to original handler if possible

When the BIOS doesn't support BBS, hooking INT 19 is the only way to add
ourselves as a boot device.  If we have to do this, we should at least
try to chain to the original INT 19 vector if our boot fails.

Idea suggested by Andrew Schran <aschran@google.com>

diff --git a/src/arch/i386/prefix/romprefix.S b/src/arch/i386/prefix/romprefix.S
index 19e6a9b380f65bebea7668b416a176db4c317f1d..1eb87c37082743408efd6986e4b7060b92646dc8 100644 (file)
--- a/src/arch/i386/prefix/romprefix.S
+++ b/src/arch/i386/prefix/romprefix.S
@@ -173,6 +173,8 @@ hook_int19:
        call    print_message
        xorw    %ax, %ax
        movw    %ax, %es
+       pushl   %es:( 0x19 * 4 )
+       popl    orig_int19
        pushw   %cs
        pushw   $int19_entry
        popl    %es:( 0x19 * 4 )
@@ -345,14 +347,24 @@ bev_entry:
 
 /* INT19 entry point
  *
- * Called via the hooked INT 19 if we detected a non-PnP BIOS.
+ * Called via the hooked INT 19 if we detected a non-PnP BIOS.  We
+ * attempt to return via the original INT 19 vector (if we were able to
+ * store it).
  */
 int19_entry:
        pushw   %cs
        call    exec
-       /* No real way to return from INT19 */
+       movl    %cs:orig_int19, %eax
+       testl   %eax, %eax
+       je      1f
+       /* Chain to original INT 19 vector */
+       ljmp    *%cs:orig_int19
+1:     /* No chained vector: issue INT 18 as a last resort */
        int     $0x18
        .size   int19_entry, . - int19_entry
+orig_int19:
+       .long   0
+       .size   orig_int19, . - orig_int19
 
 /* Execute as a boot device
  *