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====================
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Translator Internals
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====================
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QEMU is a dynamic translator. When it first encounters a piece of code,
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it converts it to the host instruction set. Usually dynamic translators
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are very complicated and highly CPU dependent. QEMU uses some tricks
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which make it relatively easily portable and simple while achieving good
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performances.
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QEMU's dynamic translation backend is called TCG, for "Tiny Code
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Generator". For more information, please take a look at ``tcg/README``.
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The following sections outline some notable features and implementation
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details of QEMU's dynamic translator.
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CPU state optimisations
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-----------------------
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The target CPUs have many internal states which change the way they
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evaluate instructions. In order to achieve a good speed, the
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translation phase considers that some state information of the virtual
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CPU cannot change in it. The state is recorded in the Translation
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Block (TB). If the state changes (e.g. privilege level), a new TB will
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be generated and the previous TB won't be used anymore until the state
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matches the state recorded in the previous TB. The same idea can be applied
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to other aspects of the CPU state. For example, on x86, if the SS,
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DS and ES segments have a zero base, then the translator does not even
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generate an addition for the segment base.
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Direct block chaining
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---------------------
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After each translated basic block is executed, QEMU uses the simulated
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Program Counter (PC) and other CPU state information (such as the CS
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segment base value) to find the next basic block.
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In its simplest, less optimized form, this is done by exiting from the
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current TB, going through the TB epilogue, and then back to the
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main loop. That’s where QEMU looks for the next TB to execute,
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translating it from the guest architecture if it isn’t already available
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in memory. Then QEMU proceeds to execute this next TB, starting at the
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prologue and then moving on to the translated instructions.
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Exiting from the TB this way will cause the ``cpu_exec_interrupt()``
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callback to be re-evaluated before executing additional instructions.
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It is mandatory to exit this way after any CPU state changes that may
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unmask interrupts.
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In order to accelerate the cases where the TB for the new
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simulated PC is already available, QEMU has mechanisms that allow
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multiple TBs to be chained directly, without having to go back to the
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main loop as described above. These mechanisms are:
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``lookup_and_goto_ptr``
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^^^^^^^^^^^^^^^^^^^^^^^
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Calling ``tcg_gen_lookup_and_goto_ptr()`` will emit a call to
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``helper_lookup_tb_ptr``. This helper will look for an existing TB that
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matches the current CPU state. If the destination TB is available its
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code address is returned, otherwise the address of the JIT epilogue is
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returned. The call to the helper is always followed by the tcg ``goto_ptr``
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opcode, which branches to the returned address. In this way, we either
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branch to the next TB or return to the main loop.
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``goto_tb + exit_tb``
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^^^^^^^^^^^^^^^^^^^^^
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The translation code usually implements branching by performing the
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following steps:
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1. Call ``tcg_gen_goto_tb()`` passing a jump slot index (either 0 or 1)
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as a parameter.
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2. Emit TCG instructions to update the CPU state with any information
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that has been assumed constant and is required by the main loop to
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correctly locate and execute the next TB. For most guests, this is
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just the PC of the branch destination, but others may store additional
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data. The information updated in this step must be inferable from both
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``cpu_get_tb_cpu_state()`` and ``cpu_restore_state()``.
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3. Call ``tcg_gen_exit_tb()`` passing the address of the current TB and
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the jump slot index again.
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Step 1, ``tcg_gen_goto_tb()``, will emit a ``goto_tb`` TCG
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instruction that later on gets translated to a jump to an address
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associated with the specified jump slot. Initially, this is the address
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of step 2's instructions, which update the CPU state information. Step 3,
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``tcg_gen_exit_tb()``, exits from the current TB returning a tagged
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pointer composed of the last executed TB’s address and the jump slot
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index.
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The first time this whole sequence is executed, step 1 simply jumps
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to step 2. Then the CPU state information gets updated and we exit from
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the current TB. As a result, the behavior is very similar to the less
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optimized form described earlier in this section.
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Next, the main loop looks for the next TB to execute using the
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current CPU state information (creating the TB if it wasn’t already
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available) and, before starting to execute the new TB’s instructions,
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patches the previously executed TB by associating one of its jump
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slots (the one specified in the call to ``tcg_gen_exit_tb()``) with the
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address of the new TB.
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The next time this previous TB is executed and we get to that same
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``goto_tb`` step, it will already be patched (assuming the destination TB
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is still in memory) and will jump directly to the first instruction of
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the destination TB, without going back to the main loop.
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For the ``goto_tb + exit_tb`` mechanism to be used, the following
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conditions need to be satisfied:
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* The change in CPU state must be constant, e.g., a direct branch and
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not an indirect branch.
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* The direct branch cannot cross a page boundary. Memory mappings
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may change, causing the code at the destination address to change.
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Note that, on step 3 (``tcg_gen_exit_tb()``), in addition to the
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jump slot index, the address of the TB just executed is also returned.
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This address corresponds to the TB that will be patched; it may be
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different than the one that was directly executed from the main loop
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if the latter had already been chained to other TBs.
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Self-modifying code and translated code invalidation
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----------------------------------------------------
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Self-modifying code is a special challenge in x86 emulation because no
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instruction cache invalidation is signaled by the application when code
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is modified.
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User-mode emulation marks a host page as write-protected (if it is
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not already read-only) every time translated code is generated for a
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basic block. Then, if a write access is done to the page, Linux raises
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a SEGV signal. QEMU then invalidates all the translated code in the page
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and enables write accesses to the page. For system emulation, write
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protection is achieved through the software MMU.
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Correct translated code invalidation is done efficiently by maintaining
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a linked list of every translated block contained in a given page. Other
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linked lists are also maintained to undo direct block chaining.
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On RISC targets, correctly written software uses memory barriers and
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cache flushes, so some of the protection above would not be
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necessary. However, QEMU still requires that the generated code always
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matches the target instructions in memory in order to handle
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exceptions correctly.
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Exception support
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-----------------
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longjmp() is used when an exception such as division by zero is
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encountered.
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The host SIGSEGV and SIGBUS signal handlers are used to get invalid
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memory accesses. QEMU keeps a map from host program counter to
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target program counter, and looks up where the exception happened
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based on the host program counter at the exception point.
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On some targets, some bits of the virtual CPU's state are not flushed to the
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memory until the end of the translation block. This is done for internal
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emulation state that is rarely accessed directly by the program and/or changes
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very often throughout the execution of a translation block---this includes
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condition codes on x86, delay slots on SPARC, conditional execution on
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Arm, and so on. This state is stored for each target instruction, and
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looked up on exceptions.
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MMU emulation
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-------------
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For system emulation QEMU uses a software MMU. In that mode, the MMU
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virtual to physical address translation is done at every memory
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access.
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QEMU uses an address translation cache (TLB) to speed up the translation.
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In order to avoid flushing the translated code each time the MMU
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mappings change, all caches in QEMU are physically indexed. This
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means that each basic block is indexed with its physical address.
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In order to avoid invalidating the basic block chain when MMU mappings
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change, chaining is only performed when the destination of the jump
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shares a page with the basic block that is performing the jump.
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The MMU can also distinguish RAM and ROM memory areas from MMIO memory
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areas. Access is faster for RAM and ROM because the translation cache also
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hosts the offset between guest address and host memory. Accessing MMIO
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memory areas instead calls out to C code for device emulation.
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Finally, the MMU helps tracking dirty pages and pages pointed to by
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translation blocks.
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