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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