xen-devel
[Xen-devel] FreeBSD boot loader on VT-x based Xen system
Folks --
I'm trying to run FreeBSD 6.0 on a VT-x based Xen system. I'm
experiencing a problem where the boot loader is crashing before the
FreeBSD kernel (proper) is loaded.
The sequence of operations is that the VMX domain is constructed, the
BIOS is executed, GRUB loads, I type the instructions to load FreeBSD
(a boot loader) and that code begins to execute and then crashes.
Here's a console log that we captured:
GNU GRUB version 0.95 (638K lower / 261112K upper memory)
[ Minimal BASH-like line editing is supported. For the first word,
TAB
lists possible command completions. Anywhere else TAB lists the
possible
completions of a device/filename.]
grub> root (hd0,1,a)
Filesystem type is ufs2, partition type 0xa5
grub> kernel /boot/loader
[FreeBSD-a.out, loadaddr=0x200000, text=0x1000, data=0x32000,
bss=0x0, entry =0x200000]
grub> boot
BTX loader 1.00 Starting in protected mode (base mem=9fc00)
Arguments passed (esp=67e88):
<howto=80000000 bootdev=a0300000 junk=0 0 0 bootinfo=67ec4>
Relocated bootinfo (size=48) to 9fbb8
Relocated arguments (size=18) to 9fba0
BTX version is 1.01
Relocated kernel (size=780) to 9000
Client base address is a000
Client format is ELF
text segment: offset=1000 vaddr=0 filesz=2a3b0 memsz=2a3b0
data segment: offset=2c000 vaddr=2c000 filesz=53cc memsz=b4c0
Loading complete
int=0000000d err=00000000 efl=00010006 eip=000090ac
eax=00094c00 ebx=00202820 ecx=00000033 edx=0000a000
esi=00209701 edi=00001f98 ebp=0000a000 esp=000017f0
cs=0008 ds=0000 es=0000 fs=0020 gs=0020 ss=0010
cs:eip=ff 35 0c 90 00 00 51 51-51 51 52 b1 07 6a 00 e2
fc 61 07 1f 0f a1 0f a9-cf fa bc 00 18 00 00 0f
ss:esp=2b 00 00 00 02 02 00 00-00 4c 09 00 33 00 00 00
0a 69 6e 74 3d 30 30 30-30 30 30 30 64 20 20 65
BTX halted
The instructions that were being executed can be found, in a FreeBSD
source tree, at:
/usr/src/sys/boot/i386/btx/btx/btx.S
Here they are for your perusal:
.code32
init.8: xorl %ecx,%ecx # Zero
movb $SEL_SDATA,%cl # To 32-bit
movw %cx,%ss # stack
/*
* Launch user task.
*/
movb $SEL_TSS,%cl # Set task
ltr %cx # register
movl $MEM_USR,%edx # User base address
movzwl %ss:BDA_MEM,%eax # Get free memory
shll 0ドルxa,%eax # To bytes
subl 0ドルx1000,%eax # Less arg space
subl %edx,%eax # Less base
movb $SEL_UDATA,%cl # User data selector
pushl %ecx # Set SS
pushl %eax # Set ESP
push 0ドルx202 # Set flags (IF set)
push $SEL_UCODE # Set CS
pushl btx_hdr+0xc # Set EIP
The error occurs with the last instruction shown above: pushl btx_hdr
+0xc.
Here's a disassembly of those instructions:
0000907f (02) 31c9 XOR ECX, ECX
00009081 (02) b1 10 MOV CL, 0x10
00009083 (02) 8ed1 MOV SS, CX
00009085 (02) b1 38 MOV CL, 0x38
00009087 (03) 0f00d9 LTR CX
0000908a (05) ba 00a00000 MOV EDX, 0xa000
0000908f (08) 36 0fb705 13040000 MOVZX EAX, SS:[0x413]
00009097 (03) c1e0 0a SHL EAX, 0xa
0000909a (05) 2d 00100000 SUB EAX, 0x1000
0000909f (02) 29d0 SUB EAX, EDX
000090a1 (02) b1 33 MOV CL, 0x33
000090a3 (01) 51 PUSH ECX
000090a4 (01) 50 PUSH EAX
000090a5 (05) 68 02020000 PUSH 0x202
000090aa (02) 6a 2b PUSH 0x2b
000090ac (06) ff35 0c000000 PUSH DWORD [0xc]
And, we can see that the instruction stream at 90ac matches the
instruction stream in the eip dump in the exception display. (Note
that I'm disassembling the btx.o file, and so the linker has not
fixed up the reference to btx_hdr+0xc to be 900c, instead we're
seeing just 0xc. Note too, that this explains why I believe I can
replace the reference to btx_hdr+0xc with an explicit reference to
the code segment register; see below.)
My guess is that there's a bug in VMXAssist. I'd like some
corroboration of that point, if possible. In fact, because the code
had long jumped to the above code sequence to enter 32bit mode, are
we still in VMXAssist? Or have we entered into pure execution mode?
It appears that the problem is based on an incorrect data segment
register value. The code segment register contains 8, which points
to a valid code segment descriptor. But, the ds register is 0, and
that's never good. How or why this code works on real hardware is
undoubtedly a side effect of ... I dunno a bug, legacy, or a design
that's not clear to me. At any rate, If I replace the push
instruction with:
pushl %cs:btx_hdr+0xc
referencing the code segment selector, then we get past the exception
noted above and into our next problem. I'm just curious about this
particular bug. Can somebody lend some insight or direction in
tracking this down? I'm more than happy to do lots of leg work on
tracking this down, but a little direction would go a long way to help.
-- Randy
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