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Signed-off-by: Tim Wiederhake <twiederh@redhat.com> Reviewed-by: Peter Krempa <pkrempa@redhat.com>
130 lines
5.5 KiB
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130 lines
5.5 KiB
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=============================
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Capturing core dumps for QEMU
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=============================
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The default behaviour for a QEMU virtual machine launched by libvirt is to
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have core dumps disabled. There can be times, however, when it is beneficial
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to collect a core dump to enable debugging.
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QEMU driver configuration
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=========================
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There is a global setting in the QEMU driver configuration file that controls
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whether core dumps are permitted, and their maximum size. Enabling core dumps
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is simply a matter of setting the maximum size to a non-zero value by editing
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the ``/etc/libvirt/qemu.conf`` file:
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::
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max_core = "unlimited"
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For an adhoc debugging session, setting the core dump size to "unlimited" is
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viable, on the assumption that core dumps will be disabled again once the
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requisite information is collected. If the intention is to leave core dumps
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permanently enabled, more careful consideration of limits is required
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Note that by default, a core dump will **NOT** include the guest RAM
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region, only memory regions used by QEMU for emulation and backend purposes.
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This is expected to be sufficient for the vast majority of debugging needs.
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When there is a need to examine guest RAM though, a further setting is
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available:
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::
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dump_guest_core = 1
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This will of course result in core dumps that are as large as the biggest
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virtual machine on the host - potentially 10's or even 100's of GB in size.
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After changing either of the settings in ``/etc/libvirt/qemu.conf`` the daemon
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hosting the QEMU driver must be restarted. For deployments using the monolithic
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daemons, this means ``libvirtd``, while for those using modular daemons this
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means ``virtqemud``:
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::
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systemctl restart libvirtd (for a monolithic deployment)
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systemctl restart virtqemud (for a modular deployment)
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While libvirt attempts to make it possible to restart the daemons without
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negatively impacting running guests, there are some management operations
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that may get interrupted. In particular long running jobs like live
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migration or block device copy jobs may abort. It is thus wise to check
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that the host is mostly idle before restarting the daemons.
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Guest core dump configuration
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=============================
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The ``dump_guest_core`` setting mentioned above will allow guest RAM to be
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included in core dumps for all virtual machines on the host. This may not
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be desirable, so it is also possible to control this on a per-virtual
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machine basis in the XML configuration:
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::
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<memory dumpCore="on">...</memory>
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Note, it is still necessary to at least set ``max_core`` to a non-zero
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value in the global configuration file.
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Some management applications may not offer the ability to customimze the
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XML configuration for a guest. In such situations, using the global
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``dump_guest_core`` setting is the only option.
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Host OS core dump storage
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=========================
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The Linux kernel default behaviour is to write core dumps to a file in the
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current working directory of the process. This will not work with QEMU
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processes launched by libvirt, because their working directory is ``/``
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which will not be writable.
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Most modern OS distros, however, now include systemd which configures a
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custom core dump handler out of the box. When this is in effect, core dumps
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from QEMU can be seen using the ``coredumpctl`` commands:
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::
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$ coredumpctl list -r
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TIME PID UID GID SIG COREFILE EXE SIZE
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Tue 2021-07-20 12:12:52 BST 2649303 107 107 SIGABRT present /usr/bin/qemu-system-x86_64 1.8M
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...snip...
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$ coredumpctl info 2649303
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PID: 2649303 (qemu-system-x86)
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UID: 107 (qemu)
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GID: 107 (qemu)
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Signal: 6 (ABRT)
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Timestamp: Tue 2021-07-20 12:12:52 BST (48min ago)
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Command Line: /usr/bin/qemu-system-x86_64 -name guest=f30,debug-threads=on ..snip... -msg timestamp=on
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Executable: /usr/bin/qemu-system-x86_64
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Control Group: /machine.slice/machine-qemu\x2d1\x2df30.scope/libvirt/emulator
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Unit: machine-qemu\x2d1\x2df30.scope
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Slice: machine.slice
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Boot ID: 6b9015d0c05f4e7fbfe4197a2c7824a2
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Machine ID: c78c8286d6d74b22ac0dd275975f9ced
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Hostname: localhost.localdomain
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Storage: /var/lib/systemd/coredump/core.qemu-system-x86.107.6b9015d0c05f4e7fbfe4197a2c7824a2.2649303.1626779572000000.zst (present)
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Disk Size: 1.8M
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Message: Process 2649303 (qemu-system-x86) of user 107 dumped core.
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Stack trace of thread 2649303:
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#0 0x00007ff3c32436be n/a (libc.so.6 + 0xf56be)
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#1 0x000055a949c0ed05 qemu_poll_ns (qemu-system-x86_64 + 0x7b0d05)
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#2 0x000055a949c0e476 main_loop_wait (qemu-system-x86_64 + 0x7b0476)
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#3 0x000055a949a36d27 qemu_main_loop (qemu-system-x86_64 + 0x5d8d27)
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#4 0x000055a94979e4d2 main (qemu-system-x86_64 + 0x3404d2)
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#5 0x00007ff3c3175b75 n/a (libc.so.6 + 0x27b75)
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#6 0x000055a9497a1f5e _start (qemu-system-x86_64 + 0x343f5e)
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Stack trace of thread 2649368:
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#0 0x00007ff3c32435bf n/a (libc.so.6 + 0xf55bf)
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#1 0x00007ff3c3af547c g_main_context_iterate.constprop.0 (libglib-2.0.so.0 + 0xa947c)
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#2 0x00007ff3c3aa0a93 g_main_loop_run (libglib-2.0.so.0 + 0x54a93)
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#3 0x00007ff3c17a727a red_worker_main.lto_priv.0 (libspice-server.so.1 + 0x5227a)
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#4 0x00007ff3c3326299 start_thread (libpthread.so.0 + 0x9299)
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#5 0x00007ff3c324e353 n/a (libc.so.6 + 0x100353)
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...snip...
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