id	summary	reporter	owner	description	type	status	priority	milestone	component	resolution	keywords	cc
11	Handle exceptions through a context switch	Nicolas Pouillon	Nicolas Pouillon	"There are two main types of exception/interrupts:
 * hardware interrupts
 * program exception (sysreq, faults, traps, ...)

We'll need to be able to switch from a task to another when handling exceptions, this is because of different needs, but in the end is wanted:
 * syscall triggering a device action, thus making the task non-eligible to scheduling until the request ends
 * syscall begin sched_switch() or yield()
 * page fault triggering a page refill

Sometimes for hardware interrupts, we want to be able to switch  tasks as well (eg timer + preempt)


Unfortunately, we cant do most of these jobs directly from the exception handlers because some CPUs (arm) use a special stack pointer when handling interrupts and exceptions. We dont want to loose time in IRQ processing, so we'll spend more time in exception processing.


We could handle all this a portable and easy way if we switched back to kernel mode (not exception or user). On exception/syscall, we can switch to other kernel-mode threads.


Optional exception:

This does not concern IRQs, which must be as-quick as possible. But as we sometimes have to switch threads on IRQ, we should add a mecanism allowing the IRQ handler to request a system service ASAP. Then directly in the asm IRQ handling code, we can detect this service request, and make the IRQ handler jump to the syscall code rather than returning to the user code.

"	enhancement	closed	minor	Preemptive scheduler usage	hexo	fixed