Re: kvm guest debug using gdb on x86

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

On Wed, May 20, 2009 at 10:10:59AM +0200, Jan Kiszka wrote:
> Aneesh Kumar K.V wrote:
> > On Wed, May 20, 2009 at 12:23:12AM +0530, Aneesh Kumar K.V wrote:
> >> Hi,
> >>
> >> With the latest qemu-kvm and 2.6.30-rc6 kernel i am not able to get
> >> the guest debugging with gdb. I get the following error.
> >>
> >> $gdb ./vmlinux
> >> GNU gdb 6.8-debian
> >> Copyright (C) 2008 Free Software Foundation, Inc.
> >> License GPLv3+: GNU GPL version 3 or later
> >> <http://gnu.org/licenses/gpl.html>
> >> This is free software: you are free to change and redistribute it.
> >> There is NO WARRANTY, to the extent permitted by law.  Type "show
> >> copying"
> >> and "show warranty" for details.
> >> This GDB was configured as "i486-linux-gnu"...
> >> (gdb) b do_fork
> >> Breakpoint 1 at 0xc106cfc8: file kernel/fork.c, line 1347.
> >> (gdb) target remote localhost:1234
> >> Remote debugging using localhost:1234
> >> [New Thread 1]
> >> Remote 'g' packet reply is too long:
> >> 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
 000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 0000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> >> (gdb)
> >>
> >> any patches that i can try ?
> > 
> > Works better with the four patches found at
> > 
> > http://git.kiszka.org/?p=kvm-userspace.git;a=shortlog;h=refs/heads/queues/gdb
> > 
> > But a next and continue doesn't get the prompt back on gdb. The guest
> > does stops the execution.
> 
> What arch is host and guest (x86-32 or -64)? What KVM versions are you
> using? Are you sure that guest and vmlinux matches?
> 

Both host and guest are x86-32.  For kvm user space I did a git clone
from git://git.kernel.org/pub/scm/virt/kvm/qemu-kvm.git. I tested with
commit 22d239bcee126742df46938ee8ddc7c6b9209e23. That version fails
with the "remote g packet reply is too long" error. Then i applied the
four patches from your git repo. That gets the guest to stop execution
on breakpoint, i can check the stack. but single stepping doesn't work.

Guest kernel I tried was linux-2.6-tip

-aneesh