1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
|
\chapter{Debugging}
Xen provides tools for debugging both Xen and guest OSes. Currently, the
Pervasive Debugger provides a GDB stub, which provides facilities for symbolic
debugging of Xen itself and of OS kernels running on top of Xen. The Trace
Buffer provides a lightweight means to log data about Xen's internal state and
behaviour at runtime, for later analysis.
\section{Pervasive Debugger}
Information on using the pervasive debugger is available in pdb.txt.
\section{Trace Buffer}
The trace buffer provides a means to observe Xen's operation from domain 0.
Trace events, inserted at key points in Xen's code, record data that can be
read by the {\tt xentrace} tool. Recording these events has a low overhead
and hence the trace buffer may be useful for debugging timing-sensitive
behaviours.
\subsection{Internal API}
To use the trace buffer functionality from within Xen, you must {\tt \#include
<xen/trace.h>}, which contains definitions related to the trace buffer. Trace
events are inserted into the buffer using the {\tt TRACE\_xD} ({\tt x} = 0, 1,
2, 3, 4 or 5) macros. These all take an event number, plus {\tt x} additional
(32-bit) data as their arguments. For trace buffer-enabled builds of Xen these
will insert the event ID and data into the trace buffer, along with the current
value of the CPU cycle-counter. For builds without the trace buffer enabled,
the macros expand to no-ops and thus can be left in place without incurring
overheads.
\subsection{Trace-enabled builds}
By default, the trace buffer is enabled only in debug builds (i.e. {\tt NDEBUG}
is not defined). It can be enabled separately by defining {\tt TRACE\_BUFFER},
either in {\tt <xen/config.h>} or on the gcc command line.
The size (in pages) of the per-CPU trace buffers can be specified using the
{\tt tbuf\_size=n } boot parameter to Xen. If the size is set to 0, the trace
buffers will be disabled.
\subsection{Dumping trace data}
When running a trace buffer build of Xen, trace data are written continuously
into the buffer data areas, with newer data overwriting older data. This data
can be captured using the {\tt xentrace} program in domain 0.
The {\tt xentrace} tool uses {\tt /dev/mem} in domain 0 to map the trace
buffers into its address space. It then periodically polls all the buffers for
new data, dumping out any new records from each buffer in turn. As a result,
for machines with multiple (logical) CPUs, the trace buffer output will not be
in overall chronological order.
The output from {\tt xentrace} can be post-processed using {\tt
xentrace\_cpusplit} (used to split trace data out into per-cpu log files) and
{\tt xentrace\_format} (used to pretty-print trace data). For the predefined
trace points, there is an example format file in {\tt tools/xentrace/formats }.
For more information, see the manual pages for {\tt xentrace}, {\tt
xentrace\_format} and {\tt xentrace\_cpusplit}.
|
