Chapter 12. Troubleshooting

This chapter provides answers to commonly asked questions. In order to improve your user experience with VirtualBox, it is recommended to read this section to learn more about common pitfalls and get recommendations on how to use the product.

12.1. Procedures and tools

12.1.1. Categorizing and isolating problems

More often than not, a virtualized guest behaves like a physical system. Any problems that a physical machine would encounter, a virtual machine will encounter as well. If, for example, Internet connectivity is lost due to external issues, virtual machines will be affected just as much as physical ones.

If a true VirtualBox problem is encountered, it helps to categorize and isolate the problem first. Here are some of the questions that should be answered before reporting a problem:

Is the problem specific to a certain guest OS? Specific release of a guest OS? Especially with Linux guest related problems, the issue may be specific to a certain distribution and version of Linux.
Is the problem specific to a certain host OS? Problems are usually not host OS specific (because most of the VirtualBox code base is shared across all supported platforms), but especially in the areas of networking and USB support, there are significant differences between host platforms. Some GUI related issues are also host specific.
Is the problem specific to certain host hardware? This category of issues is typically related to the host CPU. Because of significant differences between VT-x and AMD-V, problems may be specific to one or the other technology. The exact CPU model may also make a difference (even for software virtualization) because different CPUs support different features, which may affect certain aspects of guest CPU operation.
Is the problem specific to a certain virtualization mode? Some problems may only occur in software virtualization mode, others may be specific to hardware virtualization.
Is the problem specific to guest SMP? That is, is it related to the number of virtual CPUs (VCPUs) in the guest? Using more than one CPU usually significantly affects the internal operation of a guest OS.
Is the problem specific to the Guest Additions? In some cases, this is a given (e.g., a shared folders problem), in other cases it may be less obvious (for example, display problems). And if the problem is Guest Additions specific, is it also specific to a certain version of the Additions?
Is the problem specific to a certain environment? Some problems are related to a particular environment external to the VM; this usually involves network setup. Certain configurations of external servers such as DHCP or PXE may expose problems which do not occur with other, similar servers.
Is the problem a regression? Knowing that an issue is a regression usually makes it significantly easier to find the solution. In this case, it is crucial to know which version is affected and which is not.

12.1.2. Collecting debugging information

For problem determination, it is often important to collect debugging information which can be analyzed by VirtualBox support. This section contains information about what kind of information can be obtained.

Every time VirtualBox starts up a VM, a so-called "release log file" is created containing lots of information about the VM configuration and runtime events. The log file is called VBox.log and resides in the VM log file folder. Typically this will be a directory like this:


$HOME/VirtualBox VMs/{machinename}/Logs
When starting a VM, the configuration file of the last run will be
renamed to .1, up to
.3. Sometimes when there is a problem,
it is useful to have a look at the logs. Also when requesting support
for VirtualBox, supplying the corresponding log file is
mandatory.
For convenience, for each virtual machine, the VirtualBox main
window can show these logs in a window. To access it, select a virtual
machine from the list on the left and select "Show logs..." from the
"Machine" window.
The release log file (VBox.log) contains a wealth of diagnostic
information, such as Host OS type and version, VirtualBox version and
build (32-bit or 64-bit), a complete dump of the guest's configuration
(CFGM), detailed information about the host CPU type and supported
features, whether hardware virtualization is enabled, information about
VT-x/AMD-V setup, state transitions (creating, running, paused,
stopping, etc.), guest BIOS messages, Guest Additions messages,
device-specific log entries and, at the end of execution, final guest
state and condensed statistics.
In case of crashes, it is very important to collect crash dumps. This is true for both host and guest
crashes. For information about enabling core dumps on Linux, Solaris,
and OS X systems, refer to the core dump article on the VirtualBox
website.^[48]
You can also use VBoxManage
debugvm to create a dump of a complete virtual machine;
see Section 8.40, “VBoxManage debugvm”.
For network related problems, it is often helpful to capture a
trace of network traffic. If the traffic is routed through an adapter on
the host, it is possible to use Wireshark or a similar tool to capture
the traffic there. However, this often also includes a lot of traffic
unrelated to the VM.
VirtualBox provides an ability to capture network traffic only on
a specific VM's network adapter. Refer to the network tracing article on
the VirtualBox website^[49] for information on enabling this capture. The trace files
created by VirtualBox are in .pcap
format and can be easily analyzed with Wireshark.






12.1.3. The built-in VM debugger



VirtualBox includes a built-in VM debugger, which advanced users
may find useful. This debugger allows for examining and, to some extent,
controlling the VM state.

Warning
Use the VM debugger at your own risk. There is no support for
it, and the following documentation is only made available for
advanced users with a very high level of familiarity with the
x86/AMD64 machine instruction set, as well as detailed knowledge of
the PC architecture. A degree of familiarity with the internals of
the guest OS in question may also be very helpful.

The VM debugger is available in all regular production versions of
VirtualBox, but it is disabled by default because the average user will
have little use for it. There are two ways to access the
debugger:



A debugger console window displayed alongside the VM


Via the telnet protocol at
port 5000



The debugger can be enabled in three ways:



Start the VM directly using VirtualBox
--startvm, with an additional
--dbg,
--debug, or
--debug-command-line argument.
See the VirtualBox usage help for details.


Set the
VBOX_GUI_DBG_ENABLED or
VBOX_GUI_DBG_AUTO_SHOW
environment variable to true
before launching the VirtualBox process. Setting these variables
(only their presence is checked) is effective even when the first
VirtualBox process is the VM selector window. VMs subsequently
launched from the selector will have the debugger enabled.


Set the GUI/Dbg/Enabled
extra data item to true before
launching the VM. This can be set globally or on a per VM
basis.



A new 'Debug' menu entry will be added to the VirtualBox
application. This menu allows the user to open the debugger
console.
The VM debugger command syntax is loosely modeled on Microsoft and
IBM debuggers used on DOS, OS/2 and Windows. Users familiar with symdeb,
CodeView, or the OS/2 kernel debugger will find the VirtualBox VM
debugger familiar.
The most important command is
help. This will print brief usage help
for all debugger commands. The set of commands supported by the VM
debugger changes frequently and the
help command is always
up-to-date.
A brief summary of frequently used commands follows:



stop -- stops the VM
execution and enables single stepping


g -- continue VM
execution


t -- single step an
instruction


rg/rh/r -- print the
guest/hypervisor/current registers


kg/kh/k -- print the
guest/hypervisor/current call stack


da/db/dw/dd/dq -- print
memory contents as ASCII/bytes/words/dwords/qwords


u -- unassemble
memory


dg -- print the guest's
GDT


di -- print the guest's
IDT


dl -- print the guest's
LDT


dt -- print the guest's
TSS


dp* -- print the guest's
page table structures


bp/br -- set a
normal/recompiler breakpoint


bl -- list
breakpoints


bc -- clear a
breakpoint


writecore -- writes a VM
core file to disk, refer Section 12.1.4, “VM core format”



See the built-in help for other
available commands.
The VM debugger supports symbolic debugging, although symbols for
guest code are often not available. For Solaris guests, the
detect command automatically determines
the guest OS version and locates kernel symbols in guest's memory.
Symbolic debugging is then available. For Linux guests, the
detect commands also determines the
guest OS version, but there are no symbols in the guest's memory. Kernel
symbols are available in the file
/proc/kallsyms on Linux guests. This
file must be copied to the host, for example using
scp. The
loadmap debugger command can be used to
make the symbol information available to the VM debugger. Note that the
kallsyms file contains the symbols for
the currently loaded modules; if the guest's configuration changes, the
symbols will change as well and must be updated.
For all guests, a simple way to verify that the correct symbols
are loaded is the k command. The guest
is normally idling and it should be clear from the symbolic information
that the guest operating system's idle loop is being executed.
Another group of debugger commands is the set of
info commands. Running
info help provides complete usage
information. The information commands provide ad-hoc data pertinent to
various emulated devices and aspects of the VMM. There is no general
guideline for using the info commands,
the right command to use depends entirely on the problem being
investigated. Some of the info commands are:



cfgm -- print a branch of
the configuration tree


cpuid -- display the guest
CPUID leaves


ioport -- print registered
I/O port ranges


mmio -- print registered
MMIO ranges


mode -- print the current
paging mode


pit -- print the i8254 PIT
state


pic -- print the i8259A PIC
state


ohci/ehci/xhci -- print a subset
of the OHCI/EHCI/xHCI USB controller state


pcnet0 -- print the PCnet
state


vgatext -- print the
contents of the VGA framebuffer formatted as standard text
mode


timers -- print all VM
timers



The output of the info commands
generally requires in-depth knowledge of the emulated device and/or
VirtualBox VMM internals. However, when used properly, the information
provided can be invaluable.





12.1.4. VM core format



VirtualBox uses the 64-bit ELF format for its VM core files
created by VBoxManage debugvm; see
Section 8.40, “VBoxManage debugvm”. The VM core file contain the
memory and CPU dumps of the VM and can be useful for debugging your
guest OS. The 64-bit ELF object format specification can be obtained
here: http://downloads.openwatcom.org/ftp/devel/docs/elf-64-gen.pdf.
The overall layout of the VM core format is as follows:
[ ELF 64 Header]
[ Program Header, type PT_NOTE ]
  → offset to COREDESCRIPTOR
[ Program Header, type PT_LOAD ] - one for each contiguous physical memory range
  → Memory offset of range
  → File offset
[ Note Header, type NT_VBOXCORE ]
[ COREDESCRIPTOR ]
  → Magic
  → VM core file version
  → VBox version
  → Number of vCPUs etc.
[ Note Header, type NT_VBOXCPU ] - one for each vCPU
[ vCPU 1 Note Header ]
  [ DBGFCORECPU - vCPU 1 dump ]
[ Additional Notes + Data ] - currently unused
[ Memory dump ]
The memory descriptors contain physical addresses relative to the
guest and not virtual addresses. Regions of memory such as MMIO regions
are not included in the core file.
The relevant data structures and definitions can be found in the
VirtualBox sources under the following header files:
include/VBox/dbgfcorefmt.h,
include/iprt/x86.h and
src/VBox/Runtime/include/internal/ldrELFCommon.h.
The VM core file can be inspected using
elfdump and GNU
readelf or other similar
utilities.





^[48]http://www.virtualbox.org/wiki/Core_dump.


^[49]http://www.virtualbox.org/wiki/Network_tips.



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