1. Build with the 'simple' trace backend:
- ./configure --trace-backend=simple
+ ./configure --enable-trace-backends=simple
make
2. Create a file with the events you want to trace:
4. Pretty-print the binary trace file:
- ./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:
+Each directory in the source tree can declare a set of static trace events
+in a "trace-events" file. Each trace event declaration names the event, its
+arguments, and the format string which can be used for pretty-printing:
qemu_vmalloc(size_t size, void *ptr) "size %zu ptr %p"
qemu_vfree(void *ptr) "ptr %p"
-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:
+All "trace-events" files must be listed in the "trace-event-y" make variable
+in the top level Makefile.objs. During build the individual files are combined
+to create a "trace-events-all" file, which is processed by the "tracetool"
+script during build to generate code for the trace events. The
+"trace-events-all" file is also installed into "/usr/share/qemu".
+
+Trace events are invoked directly from source code like this:
#include "trace.h" /* needed for trace event prototype */
4. Name trace events after their function. If there are multiple trace events
in one function, append a unique distinguisher at the end of the name.
-5. If specific trace events are going to be called a huge number of times, this
- might have a noticeable performance impact even when the trace events are
- programmatically disabled. In this case you should declare the trace event
- with the "disable" property, which will effectively disable it at compile
- time (using the "nop" backend).
-
== Generic interface and monitor commands ==
-You can programmatically query and control the dynamic state of trace events
-through a backend-agnostic interface:
-
-* trace_print_events
-
-* trace_event_set_state
- Enables or disables trace events at runtime inside QEMU.
- The function returns "true" if the state of the event has been successfully
- changed, or "false" otherwise:
-
- #include "trace/control.h"
-
- trace_event_set_state("virtio_irq", true); /* enable */
- [...]
- trace_event_set_state("virtio_irq", false); /* disable */
+You can programmatically query and control the state of trace events through a
+backend-agnostic interface provided by the header "trace/control.h".
-Note that some of the backends do not provide an implementation for this
-interface, in which case QEMU will just print a warning.
+Note that some of the backends do not provide an implementation for some parts
+of this interface, in which case QEMU will just print a warning (please refer to
+header "trace/control.h" to see which routines are backend-dependent).
-This functionality is also provided through monitor commands:
+The state of events can also be queried and modified through monitor commands:
* info trace-events
View available trace events and their state. State 1 means enabled, state 0
means disabled.
* trace-event NAME on|off
- Enable/disable a given trace event.
+ Enable/disable a given trace event or a group of events (using wildcards).
The "-trace events=<file>" command line argument can be used to enable the
events listed in <file> from the very beginning of the program. This file must
contain one event name per line.
+If a line in the "-trace events=<file>" file begins with a '-', the trace event
+will be disabled instead of enabled. This is useful when a wildcard was used
+to enable an entire family of events but one noisy event needs to be disabled.
+
+Wildcard matching is supported in both the monitor command "trace-event" and the
+events list file. That means you can enable/disable the events having a common
+prefix in a batch. For example, virtio-blk trace events could be enabled using
+the following monitor command:
+
+ trace-event virtio_blk_* on
+
== Trace backends ==
The "tracetool" script automates tedious trace event code generation and also
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-backend=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 ===
-==== Monitor commands ====
+The "ftrace" backend writes trace data to ftrace marker. This effectively
+sends trace events to ftrace ring buffer, and you can compare qemu trace
+data and kernel(especially kvm.ko when using KVM) trace data.
-* info trace
- Display the contents of trace buffer. This command dumps the trace buffer
- with simple formatting. For full pretty-printing, use the simpletrace.py
- script on a binary trace file.
+if you use KVM, enable kvm events in ftrace:
- The trace buffer is written into until full. The full trace buffer is
- flushed and emptied. This means the 'info trace' will display few or no
- entries if the buffer has just been flushed.
+ # echo 1 > /sys/kernel/debug/tracing/events/kvm/enable
+
+After running qemu by root user, you can get the trace:
+
+ # cat /sys/kernel/debug/tracing/trace
+
+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>
Enable/disable/flush the trace file or set the trace file name.
==== 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:
- ./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.
monitor commands built into QEMU, instead UST utilities should be used to list,
enable/disable, and dump traces.
+Package lttng-tools is required for userspace tracing. You must ensure that the
+current user belongs to the "tracing" group, or manually launch the
+lttng-sessiond daemon for the current user prior to running any instance of
+QEMU.
+
+While running an instrumented QEMU, LTTng should be able to list all available
+events:
+
+ lttng list -u
+
+Create tracing session:
+
+ lttng create mysession
+
+Enable events:
+
+ lttng enable-event qemu:g_malloc -u
+
+Where the events can either be a comma-separated list of events, or "-a" to
+enable all tracepoint events. Start and stop tracing as needed:
+
+ lttng start
+ lttng stop
+
+View the trace:
+
+ lttng view
+
+Destroy tracing session:
+
+ lttng destroy
+
+Babeltrace can be used at any later time to view the trace:
+
+ babeltrace $HOME/lttng-traces/mysession-<date>-<time>
+
=== SystemTap ===
The "dtrace" backend uses DTrace sdt probes but has only been tested with
performed manually after a build in order to change the binary name in the .stp
probes:
- scripts/tracetool --dtrace --stap \
- --binary path/to/qemu-binary \
- --target-type system \
- --target-arch x86_64 \
- <trace-events >qemu.stp
+ scripts/tracetool.py --backends=dtrace --format=stap \
+ --binary path/to/qemu-binary \
+ --target-type system \
+ --target-name x86_64 \
+ <trace-events-all >qemu.stp
+
+== Trace event properties ==
+
+Each event in the "trace-events-all" file can be prefixed with a space-separated
+list of zero or more of the following event properties.
+
+=== "disable" ===
+
+If a specific trace event is going to be invoked a huge number of times, this
+might have a noticeable performance impact even when the event is
+programmatically disabled.
+
+In this case you should declare such event with the "disable" property. This
+will effectively disable the event at compile time (by using the "nop" backend),
+thus having no performance impact at all on regular builds (i.e., unless you
+edit the "trace-events-all" file).
+
+In addition, there might be cases where relatively complex computations must be
+performed to generate values that are only used as arguments for a trace
+function. In these cases you can use the macro 'TRACE_${EVENT_NAME}_ENABLED' to
+guard such computations and avoid its compilation when the event is disabled:
+
+ #include "trace.h" /* needed for trace event prototype */
+
+ void *qemu_vmalloc(size_t size)
+ {
+ void *ptr;
+ size_t align = QEMU_VMALLOC_ALIGN;
+
+ if (size < align) {
+ align = getpagesize();
+ }
+ ptr = qemu_memalign(align, size);
+ if (TRACE_QEMU_VMALLOC_ENABLED) { /* preprocessor macro */
+ void *complex;
+ /* some complex computations to produce the 'complex' value */
+ trace_qemu_vmalloc(size, ptr, complex);
+ }
+ return ptr;
+ }
+
+You can check both if the event has been disabled and is dynamically enabled at
+the same time using the 'trace_event_get_state' routine (see header
+"trace/control.h" for more information).
+
+=== "tcg" ===
+
+Guest code generated by TCG can be traced by defining an event with the "tcg"
+event property. Internally, this property generates two events:
+"<eventname>_trans" to trace the event at translation time, and
+"<eventname>_exec" to trace the event at execution time.
+
+Instead of using these two events, you should instead use the function
+"trace_<eventname>_tcg" during translation (TCG code generation). This function
+will automatically call "trace_<eventname>_trans", and will generate the
+necessary TCG code to call "trace_<eventname>_exec" during guest code execution.
+
+Events with the "tcg" property can be declared in the "trace-events" file with a
+mix of native and TCG types, and "trace_<eventname>_tcg" will gracefully forward
+them to the "<eventname>_trans" and "<eventname>_exec" events. Since TCG values
+are not known at translation time, these are ignored by the "<eventname>_trans"
+event. Because of this, the entry in the "trace-events" file needs two printing
+formats (separated by a comma):
+
+ tcg foo(uint8_t a1, TCGv_i32 a2) "a1=%d", "a1=%d a2=%d"
+
+For example:
+
+ #include "trace-tcg.h"
+
+ void some_disassembly_func (...)
+ {
+ uint8_t a1 = ...;
+ TCGv_i32 a2 = ...;
+ trace_foo_tcg(a1, a2);
+ }
+
+This will immediately call:
+
+ void trace_foo_trans(uint8_t a1);
+
+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 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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