[SeaBIOS] [PATCH 0/3] SeaVGABIOS serial console test

[Gerd Hoffmann]

  Hi,

> I was thinking a few instructions to test if calll worked would be
> sufficient to catch the x86emu case - something like:
> 
>   pushl $0xffffffff
>   calll 1f
> 1:popl %eax
>   cmpl %eax, $1b
>   jne running_on_broken_x86emu

Ok, finally picked this up again.

Rebased the branch, added a patch for x86emu detection.
https://www.kraxel.org/cgit/seabios/log/?h=serial

--- a/src/romlayout.S
+++ b/src/romlayout.S
@@ -531,12 +531,31 @@ entry_10_hooked:
         pushfw
         pushl %cs:sercon_int10_hook_resume
 
+       // Detect broken x86emu versions
+       pushl %eax
+       pushl %ebx
+       pushl $0xffffffff
+       calll 1f
+1:     popl %eax
+       movl $1b, %ebx
+       cmpl %eax, %ebx
+       jne running_on_broken_x86emu
+       popl %ebx  // $0xffffffff
+       popl %ebx
+       popl %eax
+
         pushl $sercon_10_splitmode
 #if CONFIG_ENTRY_EXTRASTACK
         jmp irqentry_arg_extrastack
 #else
         jmp irqentry_arg
 #endif
+running_on_broken_x86emu:
+       popw %bx
+       popl %ebx
+       popl %eax
+       // skip sercon code, jump straight to vgabios
+       iretw
 
         // int 18/19 are special - they reset stack and call into
32bit mode.
         DECLFUNC entry_19

How is x86emu broken?  I'm assuming it ignores the prefix for call, so
it'll push a 16bit return address instead of a 32bit return address to
the stack?

Patch tested on RHEL-7 and RHEL-5 (known-broken x86emu in xorg) so far.

cheers,
  Gerd