Re: [PATCH] Make bootsector stub 16-bit-only (i386)

Andrew Morton
2007-05-08T18:12:53Z

On Tue, 08 May 2007 04:25:00 -0700 "H. Peter Anvin" <hpa@zytor.com> wrote:

> Andrew Morton wrote:
>
> >>
> >> # Normalize the start address
> >> - jmpl $BOOTSEG, $start2
> >> + jmpw $BOOTSEG, $start2
> >
> > Sigh, another blow struck in the ongoing struggle between my Vaio and the
> > rest of the world.
> >
> > Stone-cold black-screen lockup immediately upon boot.
> >
> > Stock FC5 install, config at
> > http://userweb.kernel.org/~akpm/config-sony.txt
>
> Andrew, I'm seriously starting to think there is something fundamentally
> wrong with that test setup.

heh. All the other bugs have been real oh-yeah-youre-right bugs. This is
the only mystery bug which I recall.

> The bootsect code in question is never executed. AT ALL. The only
> raison d'être for it at all is to print an error message if someone
> writes the kernel to a raw floppy disk. Nor does it change the
> alignment of the header or anything else to that effect -- the assembly
> code downstream has an explicit ".org" directive. For what it's worth,
> just to make sure I'm not crazy, I just re-tested both booting the
> kernel and booting the raw disk image, in simulation and on real
> hardware, and it doesn't change anything. I used your configuration
> file (yes '' | make oldconfig) minus Bluetooth (which is broken in
> current top of Linus) against top of tree Linus + the jmpw patch. I
> obviously don't have your Vaio, but I do have my own share of quirky
> hardware. FWIW, I netbooted the hardware using pxelinux 3.50-pre7 as
> the bootloader.[*]
>
> I'm not writing this to give you a hard time, far from it. I'm
> suggesting that there might be something wrong with that rig that's
> giving you false testing failures. I don't particularly care about the
> patch itself -- all it does is save 3 bytes which are currently unused
> anyway, (although it might help Vivek's work.) However, I'm very
> concerned that you might be getting false failures, for obvious reasons.
>

I just retested bare 2.6.21 with that patch. Same hang.

Maybe the assembler or linker screwed something up.

Without patch:

(gdb) x/20i _start
0x0 <_start>: ljmpw $0x0,$0x8
0x6 <_start+6>: rolb $0x8c,(%edi)
0x9 <start2+1>: enter $0xd88e,$0x8e
0xd <start2+5>: rorb $0xfb,0x7c00bcd0(%esi)
0x14 <start2+12>: cld
0x15 <start2+13>: mov $0x20ac0031,%esi
0x1a <msg_loop+2>: (bad)
0x1b <msg_loop+3>: je 0x26 <die>
0x1d <msg_loop+5>: mov $0xe,%ah
0x1f <msg_loop+7>: mov $0x10cd0007,%ebx
0x24 <msg_loop+12>: jmp 0x18 <msg_loop>
0x26 <die>: xor %eax,%eax
0x28 <die+2>: int $0x16

With patch:

(gdb) x/20i _start
0x0 <_start>: ljmp $0xc88c,$0x7c00005
0x7 <start2+2>: mov %eax,%ds
0x9 <start2+4>: mov %eax,%es
0xb <start2+6>: mov %eax,%ss
0xd <start2+8>: mov $0xfcfb7c00,%esp
0x12 <start2+13>: mov $0x20ac002e,%esi
0x17 <msg_loop+2>: (bad)
0x18 <msg_loop+3>: je 0x23 <die>
0x1a <msg_loop+5>: mov $0xe,%ah
0x1c <msg_loop+7>: mov $0x10cd0007,%ebx
0x21 <msg_loop+12>: jmp 0x15 <msg_loop>

not sure what's going on there. We seem to have confused gdb.