On Mon, Sep 04, 2017 at 04:02:08PM +0200, Gerd Hoffmann wrote:
> > 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?
I think so. Might be safer to save/restore %esp though. This
sequence might be simpler:
pushl %eax
movl %esp, %eax
pushl $1f
retl
1: cmpl %esp, %eax
jne broken_x86emu
popl %eax
...
broken_x86emu:
movl %eax, %esp
popl %eax
iretw
Note, though, in addition to checking for broken x86emu, the assembler
entry code would also have to check that the last mode was not set via
a vesa call (to avoid the "skifree" bug).
-Kevin
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