This attempts to bring together my understanding of the requirements for icount behaviour into one reference document for our developer notes. It currently make one piece of conjecture which I think is true that we don't need gen_io_start/end statements for non-MMIO related I/O operations.
Signed-off-by: Alex Bennée <alex.ben...@linaro.org> Cc: Paolo Bonzini <pbonz...@redhat.com> Cc: Pavel Dovgalyuk <dovga...@ispras.ru> Cc: Richard Henderson <richard.hender...@linaro.org> Cc: Peter Maydell <peter.mayd...@linaro.org> Message-Id: <20200619135844.23307-1-alex.ben...@linaro.org> --- v2 - fix copyright date - it's -> its - drop mentioned of gen_io_end() - remove and correct original conjecture --- docs/devel/tcg-icount.rst | 89 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 89 insertions(+) create mode 100644 docs/devel/tcg-icount.rst diff --git a/docs/devel/tcg-icount.rst b/docs/devel/tcg-icount.rst new file mode 100644 index 00000000000..cb51cb34dde --- /dev/null +++ b/docs/devel/tcg-icount.rst @@ -0,0 +1,89 @@ +.. + Copyright (c) 2020, Linaro Limited + Written by Alex Bennée + + +======================== +TCG Instruction Counting +======================== + +TCG has long supported a feature known as icount which allows for +instruction counting during execution. This should be confused with +cycle accurate emulation - QEMU does not attempt to emulate how long +an instruction would take on real hardware. That is a job for other +more detailed (and slower) tools that simulate the rest of a +micro-architecture. + +This feature is only available for system emulation and is +incompatible with multi-threaded TCG. It can be used to better align +execution time with wall-clock time so a "slow" device doesn't run too +fast on modern hardware. It can also provides for a degree of +deterministic execution and is an essential part of the record/replay +support in QEMU. + +Core Concepts +============= + +At its heart icount is simply a count of executed instructions which +is stored in the TimersState of QEMU's timer sub-system. The number of +executed instructions can then be used to calculate QEMU_CLOCK_VIRTUAL +which represents the amount of elapsed time in the system since +execution started. Depending on the icount mode this may either be a +fixed number of ns per instructions or adjusted as execution continues +to keep wall clock time and virtual time in sync. + +To be able to calculate the number of executed instructions the +translator starts by allocating a budget of instructions to be +executed. The budget of instructions is limited by how long it will be +until the next timer will expire. We store this budget as part of a +vCPU icount_decr field which shared with the machinery for handling +cpu_exit(). The whole field is checked at the start of every +translated block and will cause a return to the outer loop to deal +with whatever caused the exit. + +In the case of icount before the flag is checked we subtract the +number of instructions the translation block would execute. If this +would cause the instruction budget to got negative we exit the main +loop and regenerate a new translation block with exactly the right +number of instructions to take the budget to 0 meaning whatever timer +was due to expire will expire exactly when we exit the main run loop. + +Dealing with MMIO +----------------- + +While we can adjust the instruction budget for known events like timer +expiry we can not do the same for MMIO. Every load/store we execute +might potentially trigger an I/O event at which point we will need an +up to date and accurate reading of the icount number. + +To deal with this case when an I/O access is made we: + + - restore un-executed instructions to the icount budget + - re-compile a single [1]_ instruction block for the current PC + - exit the cpu loop and execute the re-compiled block + +The new block is created with the CF_LAST_IO compile flag which +ensures the final instruction translation starts with a call to +gen_io_start() so we don't enter a perpetual loop constantly +recompiling a single instruction block. For translators using the +common translator_loop this is done automatically. + +.. [1] sometimes two instructions if dealing with delay slots + +Other I/O operations +-------------------- + +MMIO isn't the only type of operation for which we might need a +correct and accurate clock. IO port instructions and accesses to +system registers are the common examples here. These instructions have +to be handled by the individual translators which have the knowledge +of which operations are I/O operations. + +.. warning:: Any instruction that eventually causes an access to + QEMU_CLOCK_VIRTUAL needs to be preceded by a + gen_io_start() and must also be the last instruction + translated in the block. + + + + -- 2.20.1