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| -rw-r--r-- | examples/guestfs-performance.pod | 50 |
1 files changed, 50 insertions, 0 deletions
diff --git a/examples/guestfs-performance.pod b/examples/guestfs-performance.pod index ca4ce855..40e603b5 100644 --- a/examples/guestfs-performance.pod +++ b/examples/guestfs-performance.pod @@ -378,6 +378,54 @@ example: You will need to consult, and even modify, the source to libguestfs to fully understand the output. +=head1 DETAILED DEBUGGING USING GDB + +You can attach to the appliance BIOS/kernel using gdb. If you know +what you're doing, this can be a useful way to diagnose boot +regressions. + +Firstly, you have to change qemu so it runs with the C<-S> and C<-s> +options. These options cause qemu to pause at boot and allow you to +attach a debugger. Read L<qemu(1)> for further information. +Libguestfs invokes qemu several times (to scan the help output and so +on) and you only want the final invocation of qemu to use these +options, so use a qemu wrapper script like this: + + #!/bin/bash - + + # Set this to point to the real qemu binary. + qemu=/usr/bin/qemu-kvm + + if [ "$1" != "-global" ]; then + # Scanning help output etc. + exec $qemu "$@" + else + # Really running qemu. + exec $qemu -S -s "$@" + fi + +Now run guestfish or another libguestfs tool with the qemu wrapper +(see L<guestfs(3)/QEMU WRAPPERS> to understand what this is doing): + + LIBGUESTFS_QEMU=/path/to/qemu-wrapper guestfish -a /dev/null -v run + +This should pause just after qemu launches. In another window, attach +to qemu using gdb: + + $ gdb + (gdb) set architecture i8086 + The target architecture is assumed to be i8086 + (gdb) target remote :1234 + Remote debugging using :1234 + 0x0000fff0 in ?? () + (gdb) cont + +At this point you can use standard gdb techniques, eg. hitting C<^C> +to interrupt the boot and C<bt> get a stack trace, setting +breakpoints, etc. Note that when you are past the BIOS and into the +Linux kernel, you'll want to change the architecture back to 32 or 64 +bit. + =head1 SEE ALSO L<febootstrap(8)>, @@ -387,6 +435,8 @@ L<guestfs(3)>, L<guestfs-examples(3)>, L<libguestfs-make-fixed-appliance(1)>, L<stap(1)>, +L<qemu(1)>, +L<gdb(1)>, L<http://libguestfs.org/>. =head1 AUTHORS |