File:  [NetBSD Developer Wiki] / wikisrc / kernel_debugging_with_qemu.mdwn
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Thu Feb 19 08:32:23 2015 UTC (4 years, 11 months ago) by gson
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VMs, not VMS

    1: # Introduction
    2: 
    3: This HOWTO explains how to set up a test environment for symbolic
    4: debugging of the NetBSD kernel using a pair of QEMU virtual machines.
    5: 
    6: ## Prerequisites
    7: 
    8: You need a computer running an OS capable of cross-building NetBSD
    9: (the "host system").
   10: This can be NetBSD itself, Linux, or some other Unix-like OS.
   11: These instructions have been tested with NetBSD/amd64 6.1.4 and
   12: Debian 7 hosts.   There should be at least 20 gigabytes of available
   13: disk space.
   14: 
   15: If your host system is running NetBSD, install the following packages
   16: from pkgsrc:
   17: 
   18: * emulators/qemu >= 2.0.0nb4
   19: * misc/py-anita
   20: 
   21: If your host system uses a package system other than pkgsrc,
   22: use that to install cvs, make, gcc, qemu, the Python pexpect
   23: library, and genisoimage or mkisofs.  Also download and 
   24: install the most recent anita package from
   25: <http://www.gson.org/netbsd/anita/download/>.
   26: 
   27: ## Building the target system
   28: 
   29: Check out the NetBSD-current sources from CVS and build a full
   30: NetBSD-current/i386 release with debug symbols using the build.sh
   31: script.  The i386 port is the preferred test platform because the two
   32: other ports supported by anita are affected by known bugs: amd64 by
   33: [[PR 49276|http://gnats.NetBSD.org/49276]], and sparc by
   34: [[qemu bug 1335444|https://bugs.launchpad.net/qemu/+bug/1335444]].
   35: 
   36: If you do the build in a directory other than /usr/src, 
   37: use the -fdebug-prefix-map option to ensure that the source file names embedded
   38: in the debug symbols point to /usr/src, which is where the sources will be
   39: installed on the target system.  For example:
   40: 
   41: [[!template  id=programlisting text="""
   42:  $ CVSROOT=anoncvs@anoncvs.NetBSD.org:/cvsroot cvs checkout -A -P src
   43:  $ cd src
   44:  $ ./build.sh -j 4 -V MKDEBUG=YES -V COPTS="-g -fdebug-prefix-map=$(pwd)=/usr/src" -O ../obj -m i386 -U release sourcesets
   45: """]]
   46: 
   47: For best performance, change the number after "-j" to the number of CPU cores
   48: you have, or slightly more.
   49: 
   50: ## Installing the target system
   51: 
   52: Install the system in a virtual machine, including the debug symbols and source code:
   53: 
   54: [[!template  id=programlisting text="""
   55:  $ cd ..
   56:  $ anita --workdir work --disk-size 4G --memory-size 256M \
   57:      --sets kern-GENERIC,modules,base,etc,comp,debug,games,man,misc,tests,text,syssrc,src,sharesrc,gnusrc \
   58:      install $(pwd)/obj/releasedir/i386/
   59: """]]
   60: 
   61: ## Booting the VMs
   62: 
   63: Next, start two qemu virtual machines, one to run the kernel being
   64: debugged (the "kgdb target") and another to run gdb (the "kgdb host").
   65: 
   66: The two VMs could be run on separate physical machines, but in this
   67: example, they are run on the same physical machine and share the same
   68: hard disk image.  This sharing is made possible by the "-snapshot"
   69: option to qemu, which ensures that the disk image is not written to by
   70: either VM.
   71: 
   72: First start the kgdb target, enabling qemu's built-in GDB target stub
   73: on TCP port 1234:
   74: 
   75: [[!template  id=programlisting text="""
   76:  $ qemu-system-i386 -nographic -snapshot -hda work/wd0.img -gdb tcp::1234
   77: """]]
   78: 
   79: If you don't want everyone on the Internet to be able to debug your
   80: target, make sure incoming connections on port 1234 are blocked in
   81: your firewall.
   82: 
   83: In a second terminal window, start the kgdb host:
   84: 
   85: [[!template  id=programlisting text="""
   86:  $ qemu-system-i386 -nographic -snapshot -hda work/wd0.img
   87: """]]
   88: 
   89: Log in to the kgdb host as root and set up the network:
   90: 
   91: [[!template  id=programlisting text="""
   92:  login: root
   93:  # dhcpcd
   94: """]]
   95: 
   96: Start gdb on the kgdb host and connect to the target:
   97: 
   98: [[!template  id=programlisting text="""
   99:  # gdb /netbsd
  100:  (gdb) target remote my.host.name:1234
  101: """]]
  102: 
  103: where my.host.name is the domain name or IP address of the
  104: physical machine running the kgdb target qemu VM.
  105: 
  106: Now you should be able to get a stack trace and start debugging
  107: with full debug symbols and access to the source code:
  108: 
  109: [[!template  id=programlisting text="""
  110:  (gdb) where
  111:  (gdb) list
  112: """]]
  113: 
  114: If the stack trace prints very slowly (like 30 seconds per stack
  115: frame), it's likely because you are using a version of qemu where
  116: the user-mode networking code fails to disable the Nagle algorithm.
  117: This is fixed in the qemu in pkgsrc, but you may run into it if your
  118: qemu is not installed via pkgsrc.
  119: 
  120: ## Qemu tips
  121: 
  122: Here is a couple of useful qemu commands to know:
  123: 
  124: * Ctrl-a b will send a break which will make the NetBSD VM enter the ddb kernel debugger.
  125: 
  126: * Ctrl-a c will switch to the qemu monitor where you can enter commands like "quit" to exit qemu,
  127: or do things like saving/restoring the VM to/from a file.

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