Convert "translator internals" docs to RST, move to devel manual

Our user-facing manual currently has a section "translator internals"
which has some high-level information about the design of the
TCG translator. This should really be in our new devel/ manual.
Convert it to RST format and move it there.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190607152827.18003-2-peter.maydell@linaro.org
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 111 insertions, 0 deletions

diff --git a/docs/devel/tcg.rst b/docs/devel/tcg.rst
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+====================
+Translator Internals
+====================
+
+QEMU is a dynamic translator. When it first encounters a piece of code,
+it converts it to the host instruction set. Usually dynamic translators
+are very complicated and highly CPU dependent. QEMU uses some tricks
+which make it relatively easily portable and simple while achieving good
+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:
+
+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
+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
+be generated and the previous TB won't be used anymore until the state
+matches the state recorded in the previous TB. The same idea can be applied
+to other aspects of the CPU state.  For example, on x86, if the SS,
+DS and ES segments have a zero base, then the translator does not even
+generate an addition for the segment base.
+
+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
+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.
+
+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.
+
+Self-modifying code and translated code invalidation
+----------------------------------------------------
+
+Self-modifying code is a special challenge in x86 emulation because no
+instruction cache invalidation is signaled by the application when code
+is modified.
+
+User-mode emulation marks a host page as write-protected (if it is
+not already read-only) every time translated code is generated for a
+basic block.  Then, if a write access is done to the page, Linux raises
+a SEGV signal. QEMU then invalidates all the translated code in the page
+and enables write accesses to the page.  For system emulation, write
+protection is achieved through the software MMU.
+
+Correct translated code invalidation is done efficiently by maintaining
+a linked list of every translated block contained in a given page. Other
+linked lists are also maintained to undo direct block chaining.
+
+On RISC targets, correctly written software uses memory barriers and
+cache flushes, so some of the protection above would not be
+necessary. However, QEMU still requires that the generated code always
+matches the target instructions in memory in order to handle
+exceptions correctly.
+
+Exception support
+-----------------
+
+longjmp() is used when an exception such as division by zero is
+encountered.
+
+The host SIGSEGV and SIGBUS signal handlers are used to get invalid
+memory accesses.  QEMU keeps a map from host program counter to
+target program counter, and looks up where the exception happened
+based on the host program counter at the exception point.
+
+On some targets, some bits of the virtual CPU's state are not flushed to the
+memory until the end of the translation block.  This is done for internal
+emulation state that is rarely accessed directly by the program and/or changes
+very often throughout the execution of a translation block---this includes
+condition codes on x86, delay slots on SPARC, conditional execution on
+ARM, and so on.  This state is stored for each target instruction, and
+looked up on exceptions.
+
+MMU emulation
+-------------
+
+For system emulation QEMU uses a software MMU. In that mode, the MMU
+virtual to physical address translation is done at every memory
+access.
+
+QEMU uses an address translation cache (TLB) to speed up the translation.
+In order to avoid flushing the translated code each time the MMU
+mappings change, all caches in QEMU are physically indexed.  This
+means that each basic block is indexed with its physical address.
+
+In order to avoid invalidating the basic block chain when MMU mappings
+change, chaining is only performed when the destination of the jump
+shares a page with the basic block that is performing the jump.
+
+The MMU can also distinguish RAM and ROM memory areas from MMIO memory
+areas.  Access is faster for RAM and ROM because the translation cache also
+hosts the offset between guest address and host memory.  Accessing MMIO
+memory areas instead calls out to C code for device emulation.
+Finally, the MMU helps tracking dirty pages and pages pointed to by
+translation blocks.
+