Re: "Illegal Instruction" error in grub_script_comments

[Lennart Sorensen]

On Wed, Oct 19, 2016 at 11:32:08AM -0400, Matthew Whitehead wrote:
> Lennart,
>   I think I found the problem. I don't know what kernel.img does, but I'm 
> guessing it is an essential part of grub2, loading after the 1st and 2nd 
> stage loaders? I disassembled the binary using 'objdump -D -b binary -m i386 
> kernel.img' and found several cases both of rdtsc and cpuid. Both of these 
> instructions do not exist on first generation 80486 cpus. Here they are, and 
> I've attached the full disassembly output.
> 
>     53a0:       85 c0                   test   %eax,%eax
>     53a2:       74 6f                   je     0x5413
>     53a4:       b8 01 00 00 00          mov    $0x1,%eax
>     53a9:       0f a2                   cpuid
>     53ab:       80 e2 10                and    $0x10,%dl
>     53ae:       74 63                   je     0x5413
>     53b0:       31 c0                   xor    %eax,%eax
>     53b2:       0f a2                   cpuid
>     53b4:       0f 31                   rdtsc
>     53b6:       a3 48 67 01 00          mov    %eax,0x16748
>     53bb:       89 15 4c 67 01 00       mov    %edx,0x1674c
>     53c1:       31 c0                   xor    %eax,%eax
>     53c3:       0f a2                   cpuid
>     53c5:       0f 31                   rdtsc
>     53c7:       89 c6                   mov    %eax,%esi
>     53c9:       b8 ff ff 00 00          mov    $0xffff,%eax
>     53ce:       89 d7                   mov    %edx,%edi
>     53d0:       e8 83 00 00 00          call   0x5458
>     53d5:       31 c0                   xor    %eax,%eax
>     53d7:       0f a2                   cpuid
>     53d9:       0f 31                   rdtsc
>     53db:       89 45 e0                mov    %eax,-0x20(%ebp)
>     53de:       8b 4d e0                mov    -0x20(%ebp),%ecx
> ...
>     5439:       31 c0                   xor    %eax,%eax
>     543b:       0f a2                   cpuid
>     543d:       0f 31                   rdtsc
>     543f:       8b 0d 50 67 01 00       mov    0x16750,%ecx
>     5445:       6a 00                   push   $0x0
>     5447:       e8 8f dc ff ff          call   0x30db
> 
> How can we adjust the makefiles to include -march=i486 or something similar?

I don't think that's likely to make a difference.

The cpuid and rdtsc instructions are supposed to be protected by a check
for whether cpuid is a valid instruction, so it should never try to run
them unless that isn't working.

Would be interesting to see the failing test run under gdb in case it
can point to what instruction is illegal and on what line.

-- 
Len Sorensen