Heya,
With latest Fedora Rawhide Kernel, I see a nested KVM guest hanging at
boot (not unusual). Rich once suggested this[1] to try to attach gdb to
the nested L2 guest to find out where_ it's stuck, tonight I set out to
try it out (with KVM TCG).
Below is all what I tried.
In guest hypervisor (L1):
$ git clone git://github.com/libguestfs/libguestfs.git
$ git log | head -1
commit 82a4a8f02c5706979d961ad6c4ac767a37a3a7c9
# Activate gdb debugging code by turning its
# conditional compilation directive: s/#if 0/#if 1
$ vi src/launch.c
[. . .] # Edit, save it
Double ensure the gdb pre-processor directive is turned on:
$ grep \-S src/launch-direct.c -A4 -B2
*/
#if 1
ADD_CMDLINE (-S);
ADD_CMDLINE (-s);
warning (g, qemu debugging is enabled, connect gdb to tcp::1234 to
begin);
#endif
Compile:
$ ./autogen.sh
$ make -j4
[. . .] # Compile, address anything that comes up
$ echo $?
0
Install Kernel-debug info:
$ yum install --enablerepo=fedora-debuginfo kernel-debuginfo
Enable the 'direct' backend and invoke the appliance using the `run` script:
$ export LIBGUESTFS_BACKEND=direct
$ ./run libguestfs-test-tool
The 'qemu-kvm' command-line just hung at:
$ ./run libguestfs-test-tool
[. . .]
-chardev
socket,path=/home/tuser1/src/libguestfs/tmp/libguestfspCGc1F/guestfsd.sock,id=channel0
\
-device virtserialport,chardev=channel0,name=org.libguestfs.channel.0 \
-append 'panic=1 console=ttyS0 udevtimeout=600 no_timer_check lpj=2294686
acpi=off printk.time=1 cgroup_disable=memory root=/dev/sdb selinux=0
guestfs_verbose=1 TERM=screen'
Let's try with TCG
--
$ guestfish get_backend
direct
$ export LIBGUESTFS_BACKEND_SETTINGS=force_tcg
$ guestfish get_backend_settings
force_tcg
Run the appliance:
$ ./run libguestfs-test-tool
Again, `qemu-kvm` CLI just hung just like with KVM acceleration case
Try a couple more things
- Run `qemu-sanity-check` in L1 -- Of course, this fails too.
- Invoke (from a different shell, as root) QEMU directly with gdb
debugging options -s -S with KVM on L1:
$ qemu-system-x86_64 -s -S -nographic -nodefconfig \
-nodefaults -machine accel=kvm -m 4000 \
-drive
file=/home/tuser1/vmimages/fedora-20.qcow2,if=ide,format=qcow2,cache=none \
-serial stdio
Result: Just hung there.
Tru with TCG:
$ qemu-system-x86_64 -s -S -nographic -nodefconfig \
-nodefaults -machine accel=tcg -m 4000 \
-drive
file=/home/tuser1/vmimages/fedora-20.qcow2,if=ide,format=qcow2,cache=none \
-serial stdio
to no avail: it's just hung there, again.
What else I can try?
Did I get this right or am I making any elementary mistake here? (I'm yet
to try on a different L1 guest on a different hardware, before that I
just wanted to run this by the list.)
Version info
On L1:
$ virt-what
kvm
$ uname -r; rpm -q libvirt-daemon-kvm qemu-system-x86 \
libguestfs kernel-debuginfo
3.14.0-0.rc2.git0.1.fc21.x86_64
libvirt-daemon-kvm-1.2.1-2.fc21.x86_64
qemu-system-x86-1.7.0-4.fc21.x86_64
libguestfs-1.25.33-1.fc21.x86_64
kernel-debuginfo-3.11.10-301.fc20.x86_64
[1]
https://github.com/libguestfs/libguestfs/blob/master/src/launch-direct.c#L404
--
/kashyap
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