[PATCH v2] docs/devel: add some notes on tcg-icount for developers

[Alex Bennée]

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.bennee@linaro.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Pavel Dovgalyuk <dovgaluk@ispras.ru>
Cc: Richard Henderson <richard.henderson@linaro.org>
Cc: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200619135844.23307-1-alex.bennee@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