@@ -21,3 +21,4 @@ Contents:
testing
decodetree
secure-coding-practices
+ tcg
new file mode 100644
@@ -0,0 +1,111 @@
+====================
+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.
+
@@ -161,109 +161,6 @@ may be created from overlay with minimal amount of hand-written code.
@end itemize
-@node Translator Internals
-@section 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 @code{tcg/README}.
-
-Some notable features of QEMU's dynamic translator are:
-
-@table @strong
-
-@item 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.
-
-@item 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 @code{JUMP} opcode is
-directly patched so that the block chaining has no overhead.
-
-@item 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.
-
-@item 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.
-
-@item 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.
-@end table
-
@node QEMU compared to other emulators
@section QEMU compared to other emulators