[romprefix] Sanity-check the runtime segment address for PCI 3

Some PCI 3 BIOSes seem to provide a garbage value in %bx, which should
contain the runtime segment address.  Perform a basic sanity check: we
reject the segment if it is below the start of option ROM space.  If
the sanity check fails, we assume that the BIOS was not expecting us
to be a PCI 3 ROM, and we just leave our image in situ.

diff --git a/src/arch/i386/prefix/romprefix.S b/src/arch/i386/prefix/romprefix.S
index ca32a44..3351494 100644 (file)
--- a/src/arch/i386/prefix/romprefix.S
+++ b/src/arch/i386/prefix/romprefix.S
@@ -205,7 +205,16 @@ init:
        movb    %bl, %al
        call    print_hex_byte
        cmpb    $3, %bh
+       jb      1f
+       /* PCI >=3.0: leave %gs as-is if sane */
+       movw    %gs, %ax
+       cmpw    $0xc000, %ax
        jae     2f
+       /* PCI 3.0 with insane %gs value: print error and ignore %gs */
+       movb    $'!', %al
+       call    print_character
+       movw    %gs, %ax
+       call    print_hex_word
 1:     /* PCI <3.0: set %gs (runtime segment) = %cs (init-time segment) */
        pushw   %cs
        popw    %gs