tcg.rst (8711B)
1 ==================== 2 Translator Internals 3 ==================== 4 5 QEMU is a dynamic translator. When it first encounters a piece of code, 6 it converts it to the host instruction set. Usually dynamic translators 7 are very complicated and highly CPU dependent. QEMU uses some tricks 8 which make it relatively easily portable and simple while achieving good 9 performances. 10 11 QEMU's dynamic translation backend is called TCG, for "Tiny Code 12 Generator". For more information, please take a look at ``tcg/README``. 13 14 The following sections outline some notable features and implementation 15 details of QEMU's dynamic translator. 16 17 CPU state optimisations 18 ----------------------- 19 20 The target CPUs have many internal states which change the way they 21 evaluate instructions. In order to achieve a good speed, the 22 translation phase considers that some state information of the virtual 23 CPU cannot change in it. The state is recorded in the Translation 24 Block (TB). If the state changes (e.g. privilege level), a new TB will 25 be generated and the previous TB won't be used anymore until the state 26 matches the state recorded in the previous TB. The same idea can be applied 27 to other aspects of the CPU state. For example, on x86, if the SS, 28 DS and ES segments have a zero base, then the translator does not even 29 generate an addition for the segment base. 30 31 Direct block chaining 32 --------------------- 33 34 After each translated basic block is executed, QEMU uses the simulated 35 Program Counter (PC) and other CPU state information (such as the CS 36 segment base value) to find the next basic block. 37 38 In its simplest, less optimized form, this is done by exiting from the 39 current TB, going through the TB epilogue, and then back to the 40 main loop. That’s where QEMU looks for the next TB to execute, 41 translating it from the guest architecture if it isn’t already available 42 in memory. Then QEMU proceeds to execute this next TB, starting at the 43 prologue and then moving on to the translated instructions. 44 45 Exiting from the TB this way will cause the ``cpu_exec_interrupt()`` 46 callback to be re-evaluated before executing additional instructions. 47 It is mandatory to exit this way after any CPU state changes that may 48 unmask interrupts. 49 50 In order to accelerate the cases where the TB for the new 51 simulated PC is already available, QEMU has mechanisms that allow 52 multiple TBs to be chained directly, without having to go back to the 53 main loop as described above. These mechanisms are: 54 55 ``lookup_and_goto_ptr`` 56 ^^^^^^^^^^^^^^^^^^^^^^^ 57 58 Calling ``tcg_gen_lookup_and_goto_ptr()`` will emit a call to 59 ``helper_lookup_tb_ptr``. This helper will look for an existing TB that 60 matches the current CPU state. If the destination TB is available its 61 code address is returned, otherwise the address of the JIT epilogue is 62 returned. The call to the helper is always followed by the tcg ``goto_ptr`` 63 opcode, which branches to the returned address. In this way, we either 64 branch to the next TB or return to the main loop. 65 66 ``goto_tb + exit_tb`` 67 ^^^^^^^^^^^^^^^^^^^^^ 68 69 The translation code usually implements branching by performing the 70 following steps: 71 72 1. Call ``tcg_gen_goto_tb()`` passing a jump slot index (either 0 or 1) 73 as a parameter. 74 75 2. Emit TCG instructions to update the CPU state with any information 76 that has been assumed constant and is required by the main loop to 77 correctly locate and execute the next TB. For most guests, this is 78 just the PC of the branch destination, but others may store additional 79 data. The information updated in this step must be inferable from both 80 ``cpu_get_tb_cpu_state()`` and ``cpu_restore_state()``. 81 82 3. Call ``tcg_gen_exit_tb()`` passing the address of the current TB and 83 the jump slot index again. 84 85 Step 1, ``tcg_gen_goto_tb()``, will emit a ``goto_tb`` TCG 86 instruction that later on gets translated to a jump to an address 87 associated with the specified jump slot. Initially, this is the address 88 of step 2's instructions, which update the CPU state information. Step 3, 89 ``tcg_gen_exit_tb()``, exits from the current TB returning a tagged 90 pointer composed of the last executed TB’s address and the jump slot 91 index. 92 93 The first time this whole sequence is executed, step 1 simply jumps 94 to step 2. Then the CPU state information gets updated and we exit from 95 the current TB. As a result, the behavior is very similar to the less 96 optimized form described earlier in this section. 97 98 Next, the main loop looks for the next TB to execute using the 99 current CPU state information (creating the TB if it wasn’t already 100 available) and, before starting to execute the new TB’s instructions, 101 patches the previously executed TB by associating one of its jump 102 slots (the one specified in the call to ``tcg_gen_exit_tb()``) with the 103 address of the new TB. 104 105 The next time this previous TB is executed and we get to that same 106 ``goto_tb`` step, it will already be patched (assuming the destination TB 107 is still in memory) and will jump directly to the first instruction of 108 the destination TB, without going back to the main loop. 109 110 For the ``goto_tb + exit_tb`` mechanism to be used, the following 111 conditions need to be satisfied: 112 113 * The change in CPU state must be constant, e.g., a direct branch and 114 not an indirect branch. 115 116 * The direct branch cannot cross a page boundary. Memory mappings 117 may change, causing the code at the destination address to change. 118 119 Note that, on step 3 (``tcg_gen_exit_tb()``), in addition to the 120 jump slot index, the address of the TB just executed is also returned. 121 This address corresponds to the TB that will be patched; it may be 122 different than the one that was directly executed from the main loop 123 if the latter had already been chained to other TBs. 124 125 Self-modifying code and translated code invalidation 126 ---------------------------------------------------- 127 128 Self-modifying code is a special challenge in x86 emulation because no 129 instruction cache invalidation is signaled by the application when code 130 is modified. 131 132 User-mode emulation marks a host page as write-protected (if it is 133 not already read-only) every time translated code is generated for a 134 basic block. Then, if a write access is done to the page, Linux raises 135 a SEGV signal. QEMU then invalidates all the translated code in the page 136 and enables write accesses to the page. For system emulation, write 137 protection is achieved through the software MMU. 138 139 Correct translated code invalidation is done efficiently by maintaining 140 a linked list of every translated block contained in a given page. Other 141 linked lists are also maintained to undo direct block chaining. 142 143 On RISC targets, correctly written software uses memory barriers and 144 cache flushes, so some of the protection above would not be 145 necessary. However, QEMU still requires that the generated code always 146 matches the target instructions in memory in order to handle 147 exceptions correctly. 148 149 Exception support 150 ----------------- 151 152 longjmp() is used when an exception such as division by zero is 153 encountered. 154 155 The host SIGSEGV and SIGBUS signal handlers are used to get invalid 156 memory accesses. QEMU keeps a map from host program counter to 157 target program counter, and looks up where the exception happened 158 based on the host program counter at the exception point. 159 160 On some targets, some bits of the virtual CPU's state are not flushed to the 161 memory until the end of the translation block. This is done for internal 162 emulation state that is rarely accessed directly by the program and/or changes 163 very often throughout the execution of a translation block---this includes 164 condition codes on x86, delay slots on SPARC, conditional execution on 165 Arm, and so on. This state is stored for each target instruction, and 166 looked up on exceptions. 167 168 MMU emulation 169 ------------- 170 171 For system emulation QEMU uses a software MMU. In that mode, the MMU 172 virtual to physical address translation is done at every memory 173 access. 174 175 QEMU uses an address translation cache (TLB) to speed up the translation. 176 In order to avoid flushing the translated code each time the MMU 177 mappings change, all caches in QEMU are physically indexed. This 178 means that each basic block is indexed with its physical address. 179 180 In order to avoid invalidating the basic block chain when MMU mappings 181 change, chaining is only performed when the destination of the jump 182 shares a page with the basic block that is performing the jump. 183 184 The MMU can also distinguish RAM and ROM memory areas from MMIO memory 185 areas. Access is faster for RAM and ROM because the translation cache also 186 hosts the offset between guest address and host memory. Accessing MMIO 187 memory areas instead calls out to C code for device emulation. 188 Finally, the MMU helps tracking dirty pages and pages pointed to by 189 translation blocks. 190