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= Tracing = |
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== Introduction == |
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This document describes the tracing infrastructure in QEMU and how to use it |
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for debugging, profiling, and observing execution. |
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== Quickstart == |
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1. Build with the 'simple' trace backend: |
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./configure --trace-backend=simple |
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make |
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2. Enable trace events you are interested in: |
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$EDITOR trace-events # remove "disable" from events you want |
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3. Run the virtual machine to produce a trace file: |
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qemu ... # your normal QEMU invocation |
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4. Pretty-print the binary trace file: |
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./simpletrace.py trace-events trace-* |
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== Trace events == |
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There is a set of static trace events declared in the trace-events source |
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file. Each trace event declaration names the event, its arguments, and the |
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format string which can be used for pretty-printing: |
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qemu_malloc(size_t size, void *ptr) "size %zu ptr %p" |
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qemu_free(void *ptr) "ptr %p" |
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The trace-events file is processed by the tracetool script during build to |
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generate code for the trace events. Trace events are invoked directly from |
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source code like this: |
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#include "trace.h" /* needed for trace event prototype */ |
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void *qemu_malloc(size_t size) |
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{ |
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void *ptr; |
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if (!size && !allow_zero_malloc()) { |
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abort(); |
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} |
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ptr = oom_check(malloc(size ? size : 1)); |
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trace_qemu_malloc(size, ptr); /* <-- trace event */ |
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return ptr; |
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} |
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=== Declaring trace events === |
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The tracetool script produces the trace.h header file which is included by |
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every source file that uses trace events. Since many source files include |
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trace.h, it uses a minimum of types and other header files included to keep |
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the namespace clean and compile times and dependencies down. |
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Trace events should use types as follows: |
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* Use stdint.h types for fixed-size types. Most offsets and guest memory |
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addresses are best represented with uint32_t or uint64_t. Use fixed-size |
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types over primitive types whose size may change depending on the host |
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(32-bit versus 64-bit) so trace events don't truncate values or break |
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the build. |
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* Use void * for pointers to structs or for arrays. The trace.h header |
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cannot include all user-defined struct declarations and it is therefore |
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necessary to use void * for pointers to structs. |
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* For everything else, use primitive scalar types (char, int, long) with the |
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appropriate signedness. |
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=== Hints for adding new trace events === |
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1. Trace state changes in the code. Interesting points in the code usually |
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involve a state change like starting, stopping, allocating, freeing. State |
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changes are good trace events because they can be used to understand the |
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execution of the system. |
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2. Trace guest operations. Guest I/O accesses like reading device registers |
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are good trace events because they can be used to understand guest |
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interactions. |
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3. Use correlator fields so the context of an individual line of trace output |
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can be understood. For example, trace the pointer returned by malloc and |
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used as an argument to free. This way mallocs and frees can be matched up. |
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Trace events with no context are not very useful. |
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4. Name trace events after their function. If there are multiple trace events |
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in one function, append a unique distinguisher at the end of the name. |
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5. Declare trace events with the "disable" keyword. Some trace events can |
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produce a lot of output and users are typically only interested in a subset |
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of trace events. Marking trace events disabled by default saves the user |
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from having to manually disable noisy trace events. |
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== Trace backends == |
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The tracetool script automates tedious trace event code generation and also |
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keeps the trace event declarations independent of the trace backend. The trace |
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events are not tightly coupled to a specific trace backend, such as LTTng or |
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SystemTap. Support for trace backends can be added by extending the tracetool |
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script. |
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The trace backend is chosen at configure time and only one trace backend can |
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be built into the binary: |
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./configure --trace-backend=simple |
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For a list of supported trace backends, try ./configure --help or see below. |
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The following subsections describe the supported trace backends. |
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=== Nop === |
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The "nop" backend generates empty trace event functions so that the compiler |
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can optimize out trace events completely. This is the default and imposes no |
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performance penalty. |
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=== Simpletrace === |
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The "simple" backend supports common use cases and comes as part of the QEMU |
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source tree. It may not be as powerful as platform-specific or third-party |
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trace backends but it is portable. This is the recommended trace backend |
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unless you have specific needs for more advanced backends. |
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==== Monitor commands ==== |
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* info trace |
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Display the contents of trace buffer. This command dumps the trace buffer |
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with simple formatting. For full pretty-printing, use the simpletrace.py |
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script on a binary trace file. |
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The trace buffer is written into until full. The full trace buffer is |
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flushed and emptied. This means the 'info trace' will display few or no |
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entries if the buffer has just been flushed. |
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* info trace-events |
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View available trace events and their state. State 1 means enabled, state 0 |
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means disabled. |
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* trace-event NAME on|off |
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Enable/disable a given trace event. |
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* trace-file on|off|flush|set <path> |
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Enable/disable/flush the trace file or set the trace file name. |
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==== Enabling/disabling trace events programmatically ==== |
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The st_change_trace_event_state() function can be used to enable or disable trace |
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events at runtime inside QEMU: |
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#include "trace.h" |
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st_change_trace_event_state("virtio_irq", true); /* enable */ |
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[...] |
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st_change_trace_event_state("virtio_irq", false); /* disable */ |
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==== Analyzing trace files ==== |
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The "simple" backend produces binary trace files that can be formatted with the |
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simpletrace.py script. The script takes the trace-events file and the binary |
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trace: |
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./simpletrace.py trace-events trace-12345 |
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You must ensure that the same trace-events file was used to build QEMU, |
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otherwise trace event declarations may have changed and output will not be |
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consistent. |
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=== LTTng Userspace Tracer === |
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The "ust" backend uses the LTTng Userspace Tracer library. There are no |
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monitor commands built into QEMU, instead UST utilities should be used to list, |
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enable/disable, and dump traces. |