4. Pretty-print the binary trace file:
- ./scripts/simpletrace.py trace-events trace-*
+ ./scripts/simpletrace.py trace-events-all trace-* # Override * with QEMU <pid>
== Trace events ==
-There is a set of static trace events declared in the "trace-events" source
-file. Each trace event declaration names the event, its arguments, and the
-format string which can be used for pretty-printing:
+=== Sub-directory setup ===
- qemu_vmalloc(size_t size, void *ptr) "size %zu ptr %p"
- qemu_vfree(void *ptr) "ptr %p"
+Each directory in the source tree can declare a set of static trace events
+in a local "trace-events" file. All directories which contain "trace-events"
+files must be listed in the "trace-events-subdirs" make variable in the top
+level Makefile.objs. During build, the "trace-events" file in each listed
+subdirectory will be processed by the "tracetool" script to generate code for
+the trace events.
+
+The individual "trace-events" files are merged into a "trace-events-all" file,
+which is also installed into "/usr/share/qemu" with the name "trace-events".
+This merged file is to be used by the "simpletrace.py" script to later analyse
+traces in the simpletrace data format.
+
+In the sub-directory the following files will be automatically generated
+
+ - trace.c - the trace event state declarations
+ - trace.h - the trace event enums and probe functions
+ - trace-dtrace.h - DTrace event probe specification
+ - trace-dtrace.dtrace - DTrace event probe helper declaration
+ - trace-dtrace.o - binary DTrace provider (generated by dtrace)
+ - trace-ust.h - UST event probe helper declarations
+
+Source files in the sub-directory should #include the local 'trace.h' file,
+without any sub-directory path prefix. eg io/channel-buffer.c would do
+
+ #include "trace.h"
+
+To access the 'io/trace.h' file. While it is possible to include a trace.h
+file from outside a source files' own sub-directory, this is discouraged in
+general. It is strongly preferred that all events be declared directly in
+the sub-directory that uses them. The only exception is where there are some
+shared trace events defined in the top level directory trace-events file.
+The top level directory generates trace files with a filename prefix of
+"trace-root" instead of just "trace". This is to avoid ambiguity between
+a trace.h in the current directory, vs the top level directory.
+
+=== Using trace events ===
-The "trace-events" file is processed by the "tracetool" script during build to
-generate code for the trace events. Trace events are invoked directly from
-source code like this:
+Trace events are invoked directly from source code like this:
#include "trace.h" /* needed for trace event prototype */
special care to use PRId64 and PRIu64 for int64_t and uint64_t types,
respectively. This ensures portability between 32- and 64-bit platforms.
+Each event declaration will start with the event name, then its arguments,
+finally a format string for pretty-printing. For example:
+
+ qemu_vmalloc(size_t size, void *ptr) "size %zu ptr %p"
+ qemu_vfree(void *ptr) "ptr %p"
+
+
=== Hints for adding new trace events ===
1. Trace state changes in the code. Interesting points in the code usually
SystemTap. Support for trace backends can be added by extending the "tracetool"
script.
-The trace backend is chosen at configure time and only one trace backend can
-be built into the binary:
+The trace backends are chosen at configure time:
- ./configure --trace-backends=simple
+ ./configure --enable-trace-backends=simple
For a list of supported trace backends, try ./configure --help or see below.
+If multiple backends are enabled, the trace is sent to them all.
+
+If no backends are explicitly selected, configure will default to the
+"log" backend.
The following subsections describe the supported trace backends.
=== Nop ===
The "nop" backend generates empty trace event functions so that the compiler
-can optimize out trace events completely. This is the default and imposes no
-performance penalty.
+can optimize out trace events completely. This imposes no performance
+penalty.
Note that regardless of the selected trace backend, events with the "disable"
property will be generated with the "nop" backend.
-=== Stderr ===
+=== Log ===
-The "stderr" backend sends trace events directly to standard error. This
+The "log" backend sends trace events directly to standard error. This
effectively turns trace events into debug printfs.
This is the simplest backend and can be used together with existing code that
trace backends but it is portable. This is the recommended trace backend
unless you have specific needs for more advanced backends.
-The "simple" backend currently does not capture string arguments, it simply
-records the char* pointer value instead of the string that is pointed to.
-
=== Ftrace ===
The "ftrace" backend writes trace data to ftrace marker. This effectively
Restriction: "ftrace" backend is restricted to Linux only.
+=== Syslog ===
+
+The "syslog" backend sends trace events using the POSIX syslog API. The log
+is opened specifying the LOG_DAEMON facility and LOG_PID option (so events
+are tagged with the pid of the particular QEMU process that generated
+them). All events are logged at LOG_INFO level.
+
+NOTE: syslog may squash duplicate consecutive trace events and apply rate
+ limiting.
+
+Restriction: "syslog" backend is restricted to POSIX compliant OS.
+
==== Monitor commands ====
* trace-file on|off|flush|set <path>
==== Analyzing trace files ====
The "simple" backend produces binary trace files that can be formatted with the
-simpletrace.py script. The script takes the "trace-events" file and the binary
-trace:
+simpletrace.py script. The script takes the "trace-events-all" file and the
+binary trace:
- ./scripts/simpletrace.py trace-events trace-12345
+ ./scripts/simpletrace.py trace-events-all trace-12345
-You must ensure that the same "trace-events" file was used to build QEMU,
+You must ensure that the same "trace-events-all" file was used to build QEMU,
otherwise trace event declarations may have changed and output will not be
consistent.
performed manually after a build in order to change the binary name in the .stp
probes:
- scripts/tracetool --dtrace --stap \
- --binary path/to/qemu-binary \
- --target-type system \
- --target-name x86_64 \
- <trace-events >qemu.stp
+ scripts/tracetool.py --backends=dtrace --format=stap \
+ --binary path/to/qemu-binary \
+ --target-type system \
+ --target-name x86_64 \
+ <trace-events-all >qemu.stp
== Trace event properties ==
-Each event in the "trace-events" file can be prefixed with a space-separated
+Each event in the "trace-events-all" file can be prefixed with a space-separated
list of zero or more of the following event properties.
=== "disable" ===
In this case you should declare such event with the "disable" property. This
will effectively disable the event at compile time (by using the "nop" backend),
thus having no performance impact at all on regular builds (i.e., unless you
-edit the "trace-events" file).
+edit the "trace-events-all" file).
In addition, there might be cases where relatively complex computations must be
performed to generate values that are only used as arguments for a trace
and will generate the TCG code to call:
void trace_foo(uint8_t a1, uint32_t a2);
+
+=== "vcpu" ===
+
+Identifies events that trace vCPU-specific information. It implicitly adds a
+"CPUState*" argument, and extends the tracing print format to show the vCPU
+information. If used together with the "tcg" property, it adds a second
+"TCGv_env" argument that must point to the per-target global TCG register that
+points to the vCPU when guest code is executed (usually the "cpu_env" variable).
+
+The "tcg" and "vcpu" properties are currently only honored in the root
+./trace-events file.
+
+The following example events:
+
+ foo(uint32_t a) "a=%x"
+ vcpu bar(uint32_t a) "a=%x"
+ tcg vcpu baz(uint32_t a) "a=%x", "a=%x"
+
+Can be used as:
+
+ #include "trace-tcg.h"
+
+ CPUArchState *env;
+ TCGv_ptr cpu_env;
+
+ void some_disassembly_func(...)
+ {
+ /* trace emitted at this point */
+ trace_foo(0xd1);
+ /* trace emitted at this point */
+ trace_bar(ENV_GET_CPU(env), 0xd2);
+ /* trace emitted at this point (env) and when guest code is executed (cpu_env) */
+ trace_baz_tcg(ENV_GET_CPU(env), cpu_env, 0xd3);
+ }
+
+If the translating vCPU has address 0xc1 and code is later executed by vCPU
+0xc2, this would be an example output:
+
+ // at guest code translation
+ foo a=0xd1
+ bar cpu=0xc1 a=0xd2
+ baz_trans cpu=0xc1 a=0xd3
+ // at guest code execution
+ baz_exec cpu=0xc2 a=0xd3
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