[Qemu-devel] *BSD kernel crash at boot in qemu with seabios 1.7.4 (bisected)

[Michael Tokarev]

[Changing subject to reflect reality]

I don't really understand what it is all about, as I initially said.
But I've been told on freebsd IRC channels to post the issue to
freebsd-emulation list, which I'm Cc'ing now, and if noone there
answers, also to John Baldwin <jhb at FreeBSD>.

To bring some context back, here are a few pointers:

 http://bugs.debian.org/737142 --
   talking about pc-bsd, but it appears that the prob affects other BSD
   kernels too
 http://thread.gmane.org/gmane.comp.emulators.qemu/254074 -- this thread
 http://lists.gnu.org/archive/html/qemu-stable/2013-03/msg00037.html - a
   weird issue with freebsd reported last december

If it's a bug in *BSD kernels, maybe someone there will be glad to fix
it.  And maybe together we can come out with a more elegant solution to
this issue, or maybe to confirm (or deny) that the proposed patch is
a right thing to do.

Thanks,

/mjt

06.02.2014 08:04, Kevin O'Connor wrote:
> On Wed, Feb 05, 2014 at 12:44:06PM -0500, Kevin O'Connor wrote:
>> On Tue, Feb 04, 2014 at 04:33:19PM +0400, Michael Tokarev wrote:
>>> We have a bugreport in debian, http://bugs.debian.org/737142,
>>> stating that PC-BSD does not work with seabios-1.7.4 anymore.
>>>
>>> I digged in, and found out that it fails only with -vga std
>>> (cirrus works fine).  So I bisected the issue - only changing
>>> vgabios-stdvga.bin, and found this:
> [...]
>> This seems to be similar to the freebsd problem reported last year -
>> see:
>> http://lists.gnu.org/archive/html/qemu-stable/2013-03/msg00037.html
> [...]
>> (*) The best I can think of is to try and construct some hand crafted
>> assembler that can catch buggy x86emu emulators and force a failure..
> 
> I have put together some assember to try and run-time test for known
> broken versions of x86emu.  This patch to SeaVGABIOS does seem to
> catch the freebsd issue and in my tests it convinces freebsd to take a
> different approach.  The patch is a bit ugly though.
> 
> Comments welcome.
> -Kevin
> 
> 
> commit e3caa553e940efb6184b30a5637134c4aa9f8b65
> Author: Kevin O'Connor <kevin at koconnor.net>
> Date:   Wed Feb 5 22:47:29 2014 -0500
> 
>     vgabios: Attempt to detect old x86emu and force a fault.
>     
>     Check for cases where the leal instruction does not work.  This
>     instruction is known to not be emulated properly on old versions of
>     x86emu.  If a broken version of x86emu is found, force a fault that
>     x86emu will easily detect.  This should help prevent soft failures
>     when running old software.
>     
>     Signed-off-by: Kevin O'Connor <kevin at koconnor.net>
> 
> diff --git a/vgasrc/vgaentry.S b/vgasrc/vgaentry.S
> index 9854448..e246e7c 100644
> --- a/vgasrc/vgaentry.S
> +++ b/vgasrc/vgaentry.S
> @@ -45,9 +45,27 @@ _rom_header_signature:
>   * Entry points
>   ****************************************************************/
>  
> -        // This macro is the same as ENTRY_ARG except the "calll"
> -        // instruction is avoided to work around known issues in the
> -        // emulation of some versions of x86emu.
> +        // Force a fault if found to be running on broken x86emu versions.
> +        DECLFUNC x86emu_fault
> +x86emu_fault:
> +        int $0x03
> +1:      hlt
> +        jmp 1b
> +
> +        // This macro implements a call while avoiding instructions
> +        // that old versions of x86emu have problems with.
> +        .macro VGA_CALLL cfunc
> +        // Make sure leal instruction works.
> +        movl $0x8000, %ecx
> +        leal (%ecx, %ecx, 1), %ecx
> +        cmpl $0x10000, %ecx
> +        jne x86emu_fault
> +        // Use callw instead of calll
> +        push %ax
> +        callw \cfunc
> +        .endm
> +
> +        // This macro is the same as ENTRY_ARG except VGA_CALLL is used.
>          .macro ENTRY_ARG_VGA cfunc
>          cli
>          cld
> @@ -57,7 +75,7 @@ _rom_header_signature:
>          movl %esp, %ebx         // Backup %esp, then zero high bits
>          movzwl %sp, %esp
>          movl %esp, %eax         // First arg is pointer to struct bregs
> -        pushw %ax ; callw \cfunc
> +        VGA_CALLL \cfunc
>          movl %ebx, %esp         // Restore %esp (including high bits)
>          POPBREGS
>          .endm
> @@ -103,7 +121,7 @@ entry_10_extrastack:
>          movw %ds, %dx           // Setup %ss/%esp and call function
>          movw %dx, %ss
>          movl %eax, %esp
> -        pushw %ax ; callw handle_10
> +        VGA_CALLL handle_10
>  
>          movl %esp, %eax         // Restore registers and return
>          movw BREGS_size+4(%eax), %ss
>