1. Build with the 'simple' trace backend:
- ./configure --trace-backend=simple
+ ./configure --enable-trace-backends=simple
make
-2. Enable trace events you are interested in:
+2. Create a file with the events you want to trace:
- $EDITOR trace-events # remove "disable" from events you want
+ echo bdrv_aio_readv > /tmp/events
+ echo bdrv_aio_writev >> /tmp/events
3. Run the virtual machine to produce a trace file:
- qemu ... # your normal QEMU invocation
+ qemu -trace events=/tmp/events ... # your normal QEMU invocation
4. Pretty-print the binary trace file:
- ./simpletrace.py trace-events trace-*
+ ./scripts/simpletrace.py trace-events trace-*
== Trace events ==
-There is a set of static trace events declared in the trace-events source
+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:
- qemu_malloc(size_t size, void *ptr) "size %zu ptr %p"
- qemu_free(void *ptr) "ptr %p"
+ 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
+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:
#include "trace.h" /* needed for trace event prototype */
-
- void *qemu_malloc(size_t size)
+
+ void *qemu_vmalloc(size_t size)
{
void *ptr;
- if (!size && !allow_zero_malloc()) {
- abort();
+ size_t align = QEMU_VMALLOC_ALIGN;
+
+ if (size < align) {
+ align = getpagesize();
}
- ptr = oom_check(malloc(size ? size : 1));
- trace_qemu_malloc(size, ptr); /* <-- trace event */
+ ptr = qemu_memalign(align, size);
+ trace_qemu_vmalloc(size, ptr);
return ptr;
}
=== Declaring trace events ===
-The tracetool script produces the trace.h header file which is included by
+The "tracetool" script produces the trace.h header file which is included by
every source file that uses trace events. Since many source files include
-trace.h, it uses a minimum of types and other header files included to keep
-the namespace clean and compile times and dependencies down.
+trace.h, it uses a minimum of types and other header files included to keep the
+namespace clean and compile times and dependencies down.
Trace events should use types as follows:
Format strings should reflect the types defined in the trace event. Take
special care to use PRId64 and PRIu64 for int64_t and uint64_t types,
-respectively. This ensures portability between 32- and 64-bit platforms. Note
-that format strings must begin and end with double quotes. When using
-portability macros, ensure they are preceded and followed by double quotes:
-"value %"PRIx64"".
+respectively. This ensures portability between 32- and 64-bit platforms.
=== Hints for adding new trace events ===
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. Declare trace events with the "disable" keyword. Some trace events can
- produce a lot of output and users are typically only interested in a subset
- of trace events. Marking trace events disabled by default saves the user
- from having to manually disable noisy trace events.
+== Generic interface and monitor commands ==
+
+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 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).
+
+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 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
+The "tracetool" script automates tedious trace event code generation and also
keeps the trace event declarations independent of the trace backend. The trace
events are not tightly coupled to a specific trace backend, such as LTTng or
-SystemTap. Support for trace backends can be added by extending the tracetool
+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:
- ./configure --trace-backend=simple
+ ./configure --trace-backends=simple
For a list of supported trace backends, try ./configure --help or see below.
can optimize out trace events completely. This is the default and 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 ===
+
+The "stderr" 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
+uses DPRINTF().
+
=== Simpletrace ===
The "simple" backend supports common use cases and comes as part of the QEMU
trace backends but it is portable. This is the recommended trace backend
unless you have specific needs for more advanced backends.
-=== Stderr ===
+The "simple" backend currently does not capture string arguments, it simply
+records the char* pointer value instead of the string that is pointed to.
-The "stderr" backend sends trace events directly to standard error output
-during emulation.
+=== Ftrace ===
-==== Monitor commands ====
-
-* 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.
+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.
- 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.
+if you use KVM, enable kvm events in ftrace:
-* info trace-events
- View available trace events and their state. State 1 means enabled, state 0
- means disabled.
+ # echo 1 > /sys/kernel/debug/tracing/events/kvm/enable
-* trace-event NAME on|off
- Enable/disable a given trace event.
+After running qemu by root user, you can get the trace:
-* trace-file on|off|flush|set <path>
- Enable/disable/flush the trace file or set the trace file name.
+ # cat /sys/kernel/debug/tracing/trace
-==== Enabling/disabling trace events programmatically ====
+Restriction: "ftrace" backend is restricted to Linux only.
-The st_change_trace_event_state() function can be used to enable or disable trace
-events at runtime inside QEMU:
+==== Monitor commands ====
- #include "trace.h"
-
- st_change_trace_event_state("virtio_irq", true); /* enable */
- [...]
- st_change_trace_event_state("virtio_irq", false); /* disable */
+* 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
+simpletrace.py script. The script takes the "trace-events" file and the binary
trace:
- ./simpletrace.py trace-events trace-12345
+ ./scripts/simpletrace.py trace-events trace-12345
-You must ensure that the same trace-events file was used to build QEMU,
+You must ensure that the same "trace-events" file was used to build QEMU,
otherwise trace event declarations may have changed and output will not be
consistent.
The "ust" backend uses the LTTng Userspace Tracer library. There are no
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 $HOME/lttng-traces/mysession-<date>-<time>
+
+=== SystemTap ===
+
+The "dtrace" backend uses DTrace sdt probes but has only been tested with
+SystemTap. When SystemTap support is detected a .stp file with wrapper probes
+is generated to make use in scripts more convenient. This step can also be
+performed manually after a build in order to change the binary name in the .stp
+probes:
+
+ scripts/tracetool --dtrace --stap \
+ --binary path/to/qemu-binary \
+ --target-type system \
+ --target-name x86_64 \
+ <trace-events >qemu.stp
+
+== Trace event properties ==
+
+Each event in the "trace-events" file can be prefixed with a space-separated
+list of zero or more of the following event properties.
+
+=== "disable" ===
+
+If a specific trace event is going to be invoked a huge number of times, this
+might have a noticeable performance impact even when the event is
+programmatically disabled.
+
+In this case you should declare such event with the "disable" property. This
+will effectively disable the event at compile time (by using the "nop" backend),
+thus having no performance impact at all on regular builds (i.e., unless you
+edit the "trace-events" file).
+
+In addition, there might be cases where relatively complex computations must be
+performed to generate values that are only used as arguments for a trace
+function. In these cases you can use the macro 'TRACE_${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);
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