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docs/devel: Explain in more detail the TB chaining mechanisms

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Signed-off-by: Luis Pires <luis.pires@eldorado.org.br>
Message-Id: <20210601125143.191165-1-luis.pires@eldorado.org.br>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
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Luis Pires 2021-06-01 09:51:43 -03:00 committed by Richard Henderson
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commit a5a8b84772
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docs/devel/tcg.rst
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@ -11,13 +11,14 @@ performances.
QEMU's dynamic translation backend is called TCG, for "Tiny Code
Generator". For more information, please take a look at ``tcg/README``.
Some notable features of QEMU's dynamic translator are:
The following sections outline some notable features and implementation
details of QEMU's dynamic translator.
CPU state optimisations
-----------------------
The target CPUs have many internal states which change the way it
evaluates instructions. In order to achieve a good speed, the
The target CPUs have many internal states which change the way they
evaluate instructions. In order to achieve a good speed, the
translation phase considers that some state information of the virtual
CPU cannot change in it. The state is recorded in the Translation
Block (TB). If the state changes (e.g. privilege level), a new TB will
@ -31,17 +32,95 @@ Direct block chaining
---------------------
After each translated basic block is executed, QEMU uses the simulated
Program Counter (PC) and other cpu state information (such as the CS
Program Counter (PC) and other CPU state information (such as the CS
segment base value) to find the next basic block.
In order to accelerate the most common cases where the new simulated PC
is known, QEMU can patch a basic block so that it jumps directly to the
next one.
In its simplest, less optimized form, this is done by exiting from the
current TB, going through the TB epilogue, and then back to the
main loop. That’s where QEMU looks for the next TB to execute,
translating it from the guest architecture if it isn’t already available
in memory. Then QEMU proceeds to execute this next TB, starting at the
prologue and then moving on to the translated instructions.
The most portable code uses an indirect jump. An indirect jump makes
it easier to make the jump target modification atomic. On some host
architectures (such as x86 or PowerPC), the ``JUMP`` opcode is
directly patched so that the block chaining has no overhead.
Exiting from the TB this way will cause the ``cpu_exec_interrupt()``
callback to be re-evaluated before executing additional instructions.
It is mandatory to exit this way after any CPU state changes that may
unmask interrupts.
In order to accelerate the cases where the TB for the new
simulated PC is already available, QEMU has mechanisms that allow
multiple TBs to be chained directly, without having to go back to the
main loop as described above. These mechanisms are:
``lookup_and_goto_ptr``
^^^^^^^^^^^^^^^^^^^^^^^
Calling ``tcg_gen_lookup_and_goto_ptr()`` will emit a call to
``helper_lookup_tb_ptr``. This helper will look for an existing TB that
matches the current CPU state. If the destination TB is available its
code address is returned, otherwise the address of the JIT epilogue is
returned. The call to the helper is always followed by the tcg ``goto_ptr``
opcode, which branches to the returned address. In this way, we either
branch to the next TB or return to the main loop.
``goto_tb + exit_tb``
^^^^^^^^^^^^^^^^^^^^^
The translation code usually implements branching by performing the
following steps:
1. Call ``tcg_gen_goto_tb()`` passing a jump slot index (either 0 or 1)
as a parameter.
2. Emit TCG instructions to update the CPU state with any information
that has been assumed constant and is required by the main loop to
correctly locate and execute the next TB. For most guests, this is
just the PC of the branch destination, but others may store additional
data. The information updated in this step must be inferable from both
``cpu_get_tb_cpu_state()`` and ``cpu_restore_state()``.
3. Call ``tcg_gen_exit_tb()`` passing the address of the current TB and
the jump slot index again.
Step 1, ``tcg_gen_goto_tb()``, will emit a ``goto_tb`` TCG
instruction that later on gets translated to a jump to an address
associated with the specified jump slot. Initially, this is the address
of step 2's instructions, which update the CPU state information. Step 3,
``tcg_gen_exit_tb()``, exits from the current TB returning a tagged
pointer composed of the last executed TB’s address and the jump slot
index.
The first time this whole sequence is executed, step 1 simply jumps
to step 2. Then the CPU state information gets updated and we exit from
the current TB. As a result, the behavior is very similar to the less
optimized form described earlier in this section.
Next, the main loop looks for the next TB to execute using the
current CPU state information (creating the TB if it wasn’t already
available) and, before starting to execute the new TB’s instructions,
patches the previously executed TB by associating one of its jump
slots (the one specified in the call to ``tcg_gen_exit_tb()``) with the
address of the new TB.
The next time this previous TB is executed and we get to that same
``goto_tb`` step, it will already be patched (assuming the destination TB
is still in memory) and will jump directly to the first instruction of
the destination TB, without going back to the main loop.
For the ``goto_tb + exit_tb`` mechanism to be used, the following
conditions need to be satisfied:
* The change in CPU state must be constant, e.g., a direct branch and
not an indirect branch.
* The direct branch cannot cross a page boundary. Memory mappings
may change, causing the code at the destination address to change.
Note that, on step 3 (``tcg_gen_exit_tb()``), in addition to the
jump slot index, the address of the TB just executed is also returned.
This address corresponds to the TB that will be patched; it may be
different than the one that was directly executed from the main loop
if the latter had already been chained to other TBs.
Self-modifying code and translated code invalidation
----------------------------------------------------