definition of soft/hard interrupts.

[Lowell Gilbert]

Bahadir Balban <kirmizimantar at gmail.com> writes:

> In "The design and implementation of 4.4BSD", the execution of

I'm not sure this affects any of your particular questions, but that
book is definitely outdated at this point...

> workqueues, some timer events and scheduling are referred to as
> "software interrupts", as well as system calls. I thought only system
> calls would be in "soft interrupt" category as they are initiated with
> a software interrupt instruction.

A software interrupt actually has its own process context that runs
within the lower half of the kernel.  Device interrupts are usually
handled by packaging the event and passing it to a software interrupt
context for handling.  I don't think it has to be implemented with a
CPU software interrupt instruction.

> By my definition, a hardware interrupt is one that is notified by the
> interrupt controller, and to my knowledge, timer events are hardware
> interrupts. Am I wrong?
>
> There's also a softclock and hardclock defined. It is as if, an
> interrupt handler for an interrupt reported on the controller, is
> termed as "hard", but a low-priority workqueue initiated by a later
> timer event is called as a "software interrupt" here. The distinction
> here mainly being made by their "priority". Would you confirm this?

That's correct, but there are a couple of additional details that
might make it more clear.  Those software interrupts can be scheduled
by other device interrupts as well -- the canonical example is a
"hard" interrupt servicing a network interface card by setting up the
received packet for the software interrupt to handle later, and then
re-enabling the card for the next packet.  

> In my opinion this isn't the way to put it and software interrupts
> should only mean interrupts initiated by interrupt instructions.

I suspect the usage dates to before the invention of software
interrupt opcodes.

-- 
Lowell Gilbert, embedded/networking software engineer, Boston area
		http://be-well.ilk.org/~lowell/