Linux Process Context and SVC call in ARM

Question

As per some Linux books

kernel code that services system calls issued by user applications runs on behalf of the corresponding application process and is said to be executing in process context. Interrupt Handlers run in interrupt context.

Now svc and irq are two exceptions.

So when linux is handling svc it is in process context and while it is handling irq it is in interrupt context. Is that how it is mapped ?

Just one edit to this

It is also mentioned in books that tasklets / softirqs run in interrupt context while workqueues run in Process context. So does it mean that tasklet would run in CPSR.mode = IRQ ?

Answer 1

If I understand your confusion in the right way:

Since Linux is a capable, preemptive, complex operating system it has much finer handling of concepts such as handling of interrupts or serving software traps compared to bare metal hardware.

For example when a supervisor call (svc) happens hardware switches to SVC mode then Linux handles this as simple as preparing some data structures for handling it further then quits from SVC mode so core can continue serving in user mode thus making it possible to run into many more exception modes instead of blocking them.

It is same for IRQ mode, Linux handles bare minimum in IRQ mode. It prepares data structures as which IRQ happened, which handler should be invoked etc then exits from IRQ mode immediately to allow more to happen on that core. Later on some other internal kernel thread may process that interrupt further. Since hardware while being relatively simple runs really fast thus handling of interrupt runs in parallel with many processes.

Downside of this advanced approach is it gives no guarantees on response time requirements or overhead of it becomes visible on slower hardware like MCUs.

So ARM's exception modes provides two things for Linux: message type and priority backed with hardware support.

• Message type is what exception mode is about, if it was a SVC, IRQ, FIQ, DATA ABORT, UNDEFINED INSTRUCTION, etc. So when hardware runs into an exception mode, Linux implicitly knows what it is handling.
• Priority is about providing a capable and responsive hardware, for example system should be able to acknowledge an interrupt while handling some less important supervisor call.
• Hardware support is for handling above two easier and faster. For example some registers are banked, or there is an extra system mode to handle reentrant IRQ easier.