1.. 2 Copyright (c) 2020, Linaro Limited 3 Written by Alex Bennée 4 5 6======================== 7TCG Instruction Counting 8======================== 9 10TCG has long supported a feature known as icount which allows for 11instruction counting during execution. This should not be confused 12with cycle accurate emulation - QEMU does not attempt to emulate how 13long an instruction would take on real hardware. That is a job for 14other more detailed (and slower) tools that simulate the rest of a 15micro-architecture. 16 17This feature is only available for system emulation and is 18incompatible with multi-threaded TCG. It can be used to better align 19execution time with wall-clock time so a "slow" device doesn't run too 20fast on modern hardware. It can also provides for a degree of 21deterministic execution and is an essential part of the record/replay 22support in QEMU. 23 24Core Concepts 25============= 26 27At its heart icount is simply a count of executed instructions which 28is stored in the TimersState of QEMU's timer sub-system. The number of 29executed instructions can then be used to calculate QEMU_CLOCK_VIRTUAL 30which represents the amount of elapsed time in the system since 31execution started. Depending on the icount mode this may either be a 32fixed number of ns per instruction or adjusted as execution continues 33to keep wall clock time and virtual time in sync. 34 35To be able to calculate the number of executed instructions the 36translator starts by allocating a budget of instructions to be 37executed. The budget of instructions is limited by how long it will be 38until the next timer will expire. We store this budget as part of a 39vCPU icount_decr field which shared with the machinery for handling 40cpu_exit(). The whole field is checked at the start of every 41translated block and will cause a return to the outer loop to deal 42with whatever caused the exit. 43 44In the case of icount, before the flag is checked we subtract the 45number of instructions the translation block would execute. If this 46would cause the instruction budget to go negative we exit the main 47loop and regenerate a new translation block with exactly the right 48number of instructions to take the budget to 0 meaning whatever timer 49was due to expire will expire exactly when we exit the main run loop. 50 51Dealing with MMIO 52----------------- 53 54While we can adjust the instruction budget for known events like timer 55expiry we cannot do the same for MMIO. Every load/store we execute 56might potentially trigger an I/O event, at which point we will need an 57up to date and accurate reading of the icount number. 58 59To deal with this case, when an I/O access is made we: 60 61 - restore un-executed instructions to the icount budget 62 - re-compile a single [1]_ instruction block for the current PC 63 - exit the cpu loop and execute the re-compiled block 64 65The new block is created with the CF_LAST_IO compile flag which 66ensures the final instruction translation starts with a call to 67gen_io_start() so we don't enter a perpetual loop constantly 68recompiling a single instruction block. For translators using the 69common translator_loop this is done automatically. 70 71.. [1] sometimes two instructions if dealing with delay slots 72 73Other I/O operations 74-------------------- 75 76MMIO isn't the only type of operation for which we might need a 77correct and accurate clock. IO port instructions and accesses to 78system registers are the common examples here. These instructions have 79to be handled by the individual translators which have the knowledge 80of which operations are I/O operations. 81 82When the translator is handling an instruction of this kind: 83 84* it must call gen_io_start() if icount is enabled, at some 85 point before the generation of the code which actually does 86 the I/O, using a code fragment similar to: 87 88.. code:: c 89 90 if (tb_cflags(s->base.tb) & CF_USE_ICOUNT) { 91 gen_io_start(); 92 } 93 94* it must end the TB immediately after this instruction 95 96Note that some older front-ends call a "gen_io_end()" function: 97this is obsolete and should not be used. 98