[Qemu-devel] [Bug 640213] [NEW] QEMU does not communicate properly with GDB with a 64 bit guest

[Ted Harkington]

Public bug reported:

I have been trying to figure out why I cannot debug a 64 bit kernel of
my own invention.

I launch qemu-system-x86_64 with the -s -S flags, we also specify -cpu
core2duo -vga std and a -hda with an ext2 FS holding our multiboot
kernel and GRUB2.

When I try to set breakpoints and "continue" in GDB (7.2) using the very
latest HEAD (b6601141cd2a170dfe773987b06f716a190ea7e0) or 0.12.0 or
0.12.5 or 13.0.rc0 or 13.0.rc1, I get failures of the same nature:

0x0000000000000000 in ?? ()
(gdb) break main
Breakpoint 1 at 0x101730: file src/kernel/init.c, line 18.
(gdb) c

Program received signal SIGTRAP, Trace/breakpoint trap.
0x0000000000000000 in ?? ()
(gdb)

Note that in this case, main lies in 64 bit mode. However, trying to
break on _start yields virtually the same effect and _start is 32 bit
code.

By doing a git bisect, I managed to narrow the commit that introduced this bug 
to 5f30fa18ad043a841fe9f0c3917ac60f2519ebd1. Reverting this commit on HEAD 
seemingly fixed the problem for both the 32 bit and 64 bit cases.
I might be doing something incorrectly on my end but this seemed to fix the 
problem.

Perhaps the pertinent thing to do would be to revert
5f30fa18ad043a841fe9f0c3917ac60f2519ebd1 as it seems to do nothing but
break things unless, of course, this would only break something that I
am not aware of further.

** Affects: qemu
     Importance: Undecided
         Status: New

-- 
QEMU does not communicate properly with GDB with a 64 bit guest
https://bugs.launchpad.net/bugs/640213
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Status in QEMU: New

Bug description:
I have been trying to figure out why I cannot debug a 64 bit kernel of my own 
invention.

I launch qemu-system-x86_64 with the -s -S flags, we also specify -cpu core2duo 
-vga std and a -hda with an ext2 FS holding our multiboot kernel and GRUB2.

When I try to set breakpoints and "continue" in GDB (7.2) using the very latest 
HEAD (b6601141cd2a170dfe773987b06f716a190ea7e0) or 0.12.0 or 0.12.5 or 13.0.rc0 
or 13.0.rc1, I get failures of the same nature:

0x0000000000000000 in ?? ()
(gdb) break main
Breakpoint 1 at 0x101730: file src/kernel/init.c, line 18.
(gdb) c

Program received signal SIGTRAP, Trace/breakpoint trap.
0x0000000000000000 in ?? ()
(gdb)

Note that in this case, main lies in 64 bit mode. However, trying to break on 
_start yields virtually the same effect and _start is 32 bit code.

By doing a git bisect, I managed to narrow the commit that introduced this bug 
to 5f30fa18ad043a841fe9f0c3917ac60f2519ebd1. Reverting this commit on HEAD 
seemingly fixed the problem for both the 32 bit and 64 bit cases.
I might be doing something incorrectly on my end but this seemed to fix the 
problem.

Perhaps the pertinent thing to do would be to revert 
5f30fa18ad043a841fe9f0c3917ac60f2519ebd1 as it seems to do nothing but break 
things unless, of course, this would only break something that I am not aware 
of further.