There is no need to save the thread pointer in _CPU_Context_switch()
since it is a thread invariant. It is initialized once in
_CPU_Context_Initialize().
Use r8 instead of r5 to slightly optimize _CPU_Context_switch(). It is
not a big deal, however, we already assume r12 is used by
_CPU_Context_switch(). Treat r5 the in same way.
Use a processor-specific interrupt frame during context switches in case
the executing thread is longer executes on the processor and the heir
thread is about to start execution. During this period we must not use
a thread stack for interrupt processing.
Update #2809.
According to the C11 and C++11 memory models only a read-modify-write
operation guarantees that we read the last value written in modification
order. Avoid the sequential consistent thread fence and instead use the
inter-processor interrupt to set the thread dispatch necessary
indicator.
Add AltiVec and FPU support to the Context_Control in case we use the
e6500 multilib.
Add PPC_MULTILIB_ALTIVEC and PPC_MULTILIB_FPU multilib defines. Add
non-volatile AltiVec and FPU context to Context_Control. Add save/restore of
non-volatile AltiVec and FPU to _CPU_Context_switch(). Add save/restore
of volatile AltiVec and FPU context to the exception code. Adjust data
cache optimizations for the new context and cache line size.
Fix context switch on SMP for ARM, PowerPC and SPARC.
Atomically test and set the is executing indicator of the heir context
to ensure that at most one processor uses the heir context. Break the
busy wait loop also due to heir updates.
We must not alter the is executing indicator in
_CPU_Context_Initialize() since this would cause an invalid state during
a self restart.
The is executing indicator must be valid at creation time since
otherwise _Thread_Kill_zombies() uses an undefined value for not started
threads. This could result in a system life lock.
The current implementation of task migration in RTEMS has some
implications with respect to the interrupt latency. It is crucial to
preserve the system invariant that a task can execute on at most one
processor in the system at a time. This is accomplished with a boolean
indicator in the task context. The processor architecture specific
low-level task context switch code will mark that a task context is no
longer executing and waits that the heir context stopped execution
before it restores the heir context and resumes execution of the heir
task. So there is one point in time in which a processor is without a
task. This is essential to avoid cyclic dependencies in case multiple
tasks migrate at once. Otherwise some supervising entity is necessary to
prevent life-locks. Such a global supervisor would lead to scalability
problems so this approach is not used. Currently the thread dispatch is
performed with interrupts disabled. So in case the heir task is
currently executing on another processor then this prolongs the time of
disabled interrupts since one processor has to wait for another
processor to make progress.
It is difficult to avoid this issue with the interrupt latency since
interrupts normally store the context of the interrupted task on its
stack. In case a task is marked as not executing we must not use its
task stack to store such an interrupt context. We cannot use the heir
stack before it stopped execution on another processor. So if we enable
interrupts during this transition we have to provide an alternative task
independent stack for this time frame. This issue needs further
investigation.
PowerPC cores with the SPE (Signal Processing Extension) have 64-bit
general-purpose registers. The SPE context switch code has been merged
with the standard context switch code. The context switch may use cache
operations to increase the performance. It will be ensured that the
context is 32-byte aligned (PPC_DEFAULT_CACHE_LINE_SIZE). This
increases the overall memory size of the context area in the thread
control block slightly. The general-purpose registers GPR2 and GPR13
are no longer part of the context. The BSP must initialize these
registers during startup (usually initialized by the __eabi() function).
The new BSP option BSP_USE_DATA_CACHE_BLOCK_TOUCH can be used to enable
the dcbt instruction in the context switch.
The new BSP option BSP_USE_SYNC_IN_CONTEXT_SWITCH can be used to enable
sync and isync instructions in the context switch. This should be not
necessary in most cases.
Script does what is expected and tries to do it as
smartly as possible.
+ remove occurrences of two blank comment lines
next to each other after Id string line removed.
+ remove entire comment blocks which only exited to
contain CVS Ids
+ If the processing left a blank line at the top of
a file, it was removed.
* new-exceptions/cpu_asm.S: the book says a context
synchronizing instruction (isync) is necessary after flipping
certain bits (e.g, MSR_FP) in msr -- since this could happen as
part of a context switch I added 'isync'.
* new_exception_processing/cpu_asm.S: Support double or single
precision context switches. Note that doing a single precision
context save/restore on a double precision PowerPC machine does not
only result in rounding errors but also screws up the FPSCR register!
