QEMU TCG Plugins

QEMU TCG plugins provide a way for users to run experiments taking advantage of the total system control emulation can have over a guest. It provides a mechanism for plugins to subscribe to events during translation and execution and optionally callback into the plugin during these events. TCG plugins are unable to change the system state only monitor it passively. However they can do this down to an individual instruction granularity including potentially subscribing to all load and store operations.

API Stability

This is a new feature for QEMU and it does allow people to develop out-of-tree plugins that can be dynamically linked into a running QEMU process. However the project reserves the right to change or break the API should it need to do so. The best way to avoid this is to submit your plugin upstream so they can be updated if/when the API changes.

API versioning

All plugins need to declare a symbol which exports the plugin API version they were built against. This can be done simply by:

QEMU_PLUGIN_EXPORT int qemu_plugin_version = QEMU_PLUGIN_VERSION;

The core code will refuse to load a plugin that doesn’t export a qemu_plugin_version symbol or if plugin version is outside of QEMU’s supported range of API versions.

Additionally the qemu_info_t structure which is passed to the qemu_plugin_install method of a plugin will detail the minimum and current API versions supported by QEMU. The API version will be incremented if new APIs are added. The minimum API version will be incremented if existing APIs are changed or removed.

Exposure of QEMU internals

The plugin architecture actively avoids leaking implementation details about how QEMU’s translation works to the plugins. While there are conceptions such as translation time and translation blocks the details are opaque to plugins. The plugin is able to query select details of instructions and system configuration only through the exported qemu_plugin functions.

Query Handle Lifetime

Each callback provides an opaque anonymous information handle which can usually be further queried to find out information about a translation, instruction or operation. The handles themselves are only valid during the lifetime of the callback so it is important that any information that is needed is extracted during the callback and saved by the plugin.

Usage

The QEMU binary needs to be compiled for plugin support:

configure --enable-plugins

Once built a program can be run with multiple plugins loaded each with their own arguments:

$QEMU $OTHER_QEMU_ARGS \
    -plugin tests/plugin/libhowvec.so,arg=inline,arg=hint \
    -plugin tests/plugin/libhotblocks.so

Arguments are plugin specific and can be used to modify their behaviour. In this case the howvec plugin is being asked to use inline ops to count and break down the hint instructions by type.

Plugin Life cycle

First the plugin is loaded and the public qemu_plugin_install function is called. The plugin will then register callbacks for various plugin events. Generally plugins will register a handler for the atexit if they want to dump a summary of collected information once the program/system has finished running.

When a registered event occurs the plugin callback is invoked. The callbacks may provide additional information. In the case of a translation event the plugin has an option to enumerate the instructions in a block of instructions and optionally register callbacks to some or all instructions when they are executed.

There is also a facility to add an inline event where code to increment a counter can be directly inlined with the translation. Currently only a simple increment is supported. This is not atomic so can miss counts. If you want absolute precision you should use a callback which can then ensure atomicity itself.

Finally when QEMU exits all the registered atexit callbacks are invoked.

Internals

Locking

We have to ensure we cannot deadlock, particularly under MTTCG. For this we acquire a lock when called from plugin code. We also keep the list of callbacks under RCU so that we do not have to hold the lock when calling the callbacks. This is also for performance, since some callbacks (e.g. memory access callbacks) might be called very frequently.

  • A consequence of this is that we keep our own list of CPUs, so that we do not have to worry about locking order wrt cpu_list_lock.
  • Use a recursive lock, since we can get registration calls from callbacks.

As a result registering/unregistering callbacks is “slow”, since it takes a lock. But this is very infrequent; we want performance when calling (or not calling) callbacks, not when registering them. Using RCU is great for this.

We support the uninstallation of a plugin at any time (e.g. from plugin callbacks). This allows plugins to remove themselves if they no longer want to instrument the code. This operation is asynchronous which means callbacks may still occur after the uninstall operation is requested. The plugin isn’t completely uninstalled until the safe work has executed while all vCPUs are quiescent.

Example Plugins

There are a number of plugins included with QEMU and you are encouraged to contribute your own plugins plugins upstream. There is a contrib/plugins directory where they can go.

  • tests/plugins

These are some basic plugins that are used to test and exercise the API during the make check-tcg target.

  • contrib/plugins/hotblocks.c

The hotblocks plugin allows you to examine the where hot paths of execution are in your program. Once the program has finished you will get a sorted list of blocks reporting the starting PC, translation count, number of instructions and execution count. This will work best with linux-user execution as system emulation tends to generate re-translations as blocks from different programs get swapped in and out of system memory.

If your program is single-threaded you can use the inline option for slightly faster (but not thread safe) counters.

Example:

./aarch64-linux-user/qemu-aarch64 \
  -plugin contrib/plugins/libhotblocks.so -d plugin \
  ./tests/tcg/aarch64-linux-user/sha1
SHA1=15dd99a1991e0b3826fede3deffc1feba42278e6
collected 903 entries in the hash table
pc, tcount, icount, ecount
0x0000000041ed10, 1, 5, 66087
0x000000004002b0, 1, 4, 66087
...
  • contrib/plugins/hotpages.c

Similar to hotblocks but this time tracks memory accesses:

./aarch64-linux-user/qemu-aarch64 \
  -plugin contrib/plugins/libhotpages.so -d plugin \
  ./tests/tcg/aarch64-linux-user/sha1
SHA1=15dd99a1991e0b3826fede3deffc1feba42278e6
Addr, RCPUs, Reads, WCPUs, Writes
0x000055007fe000, 0x0001, 31747952, 0x0001, 8835161
0x000055007ff000, 0x0001, 29001054, 0x0001, 8780625
0x00005500800000, 0x0001, 687465, 0x0001, 335857
0x0000000048b000, 0x0001, 130594, 0x0001, 355
0x0000000048a000, 0x0001, 1826, 0x0001, 11
  • contrib/plugins/howvec.c

This is an instruction classifier so can be used to count different types of instructions. It has a number of options to refine which get counted. You can give an argument for a class of instructions to break it down fully, so for example to see all the system registers accesses:

./aarch64-softmmu/qemu-system-aarch64 $(QEMU_ARGS) \
  -append "root=/dev/sda2 systemd.unit=benchmark.service" \
  -smp 4 -plugin ./contrib/plugins/libhowvec.so,arg=sreg -d plugin

which will lead to a sorted list after the class breakdown:

Instruction Classes:
Class:   UDEF                   not counted
Class:   SVE                    (68 hits)
Class:   PCrel addr             (47789483 hits)
Class:   Add/Sub (imm)          (192817388 hits)
Class:   Logical (imm)          (93852565 hits)
Class:   Move Wide (imm)        (76398116 hits)
Class:   Bitfield               (44706084 hits)
Class:   Extract                (5499257 hits)
Class:   Cond Branch (imm)      (147202932 hits)
Class:   Exception Gen          (193581 hits)
Class:     NOP                  not counted
Class:   Hints                  (6652291 hits)
Class:   Barriers               (8001661 hits)
Class:   PSTATE                 (1801695 hits)
Class:   System Insn            (6385349 hits)
Class:   System Reg             counted individually
Class:   Branch (reg)           (69497127 hits)
Class:   Branch (imm)           (84393665 hits)
Class:   Cmp & Branch           (110929659 hits)
Class:   Tst & Branch           (44681442 hits)
Class:   AdvSimd ldstmult       (736 hits)
Class:   ldst excl              (9098783 hits)
Class:   Load Reg (lit)         (87189424 hits)
Class:   ldst noalloc pair      (3264433 hits)
Class:   ldst pair              (412526434 hits)
Class:   ldst reg (imm)         (314734576 hits)
Class: Loads & Stores           (2117774 hits)
Class: Data Proc Reg            (223519077 hits)
Class: Scalar FP                (31657954 hits)
Individual Instructions:
Instr: mrs x0, sp_el0           (2682661 hits)  (op=0xd5384100/  System Reg)
Instr: mrs x1, tpidr_el2        (1789339 hits)  (op=0xd53cd041/  System Reg)
Instr: mrs x2, tpidr_el2        (1513494 hits)  (op=0xd53cd042/  System Reg)
Instr: mrs x0, tpidr_el2        (1490823 hits)  (op=0xd53cd040/  System Reg)
Instr: mrs x1, sp_el0           (933793 hits)   (op=0xd5384101/  System Reg)
Instr: mrs x2, sp_el0           (699516 hits)   (op=0xd5384102/  System Reg)
Instr: mrs x4, tpidr_el2        (528437 hits)   (op=0xd53cd044/  System Reg)
Instr: mrs x30, ttbr1_el1       (480776 hits)   (op=0xd538203e/  System Reg)
Instr: msr ttbr1_el1, x30       (480713 hits)   (op=0xd518203e/  System Reg)
Instr: msr vbar_el1, x30        (480671 hits)   (op=0xd518c01e/  System Reg)
...

To find the argument shorthand for the class you need to examine the source code of the plugin at the moment, specifically the *opt argument in the InsnClassExecCount tables.

  • contrib/plugins/lockstep.c

This is a debugging tool for developers who want to find out when and where execution diverges after a subtle change to TCG code generation. It is not an exact science and results are likely to be mixed once asynchronous events are introduced. While the use of -icount can introduce determinism to the execution flow it doesn’t always follow the translation sequence will be exactly the same. Typically this is caused by a timer firing to service the GUI causing a block to end early. However in some cases it has proved to be useful in pointing people at roughly where execution diverges. The only argument you need for the plugin is a path for the socket the two instances will communicate over:

./sparc-softmmu/qemu-system-sparc -monitor none -parallel none \
  -net none -M SS-20 -m 256 -kernel day11/zImage.elf \
  -plugin ./contrib/plugins/liblockstep.so,arg=lockstep-sparc.sock \
-d plugin,nochain

which will eventually report:

qemu-system-sparc: warning: nic lance.0 has no peer
@ 0x000000ffd06678 vs 0x000000ffd001e0 (2/1 since last)
@ 0x000000ffd07d9c vs 0x000000ffd06678 (3/1 since last)
Δ insn_count @ 0x000000ffd07d9c (809900609) vs 0x000000ffd06678 (809900612)
  previously @ 0x000000ffd06678/10 (809900609 insns)
  previously @ 0x000000ffd001e0/4 (809900599 insns)
  previously @ 0x000000ffd080ac/2 (809900595 insns)
  previously @ 0x000000ffd08098/5 (809900593 insns)
  previously @ 0x000000ffd080c0/1 (809900588 insns)