Matthew Dillon wrote:
> Oops, let me add an addendum, I didn't answer the original question!
>
> :(Julian wrote...)
> :
> :Am I also right in assuming that all the registers that the user was
> :running when they did the KERNCALL have been saved on the KERNEL stack by
> :the time that the above routines are called?
> :
> :(It's a pitty because if they were saved on the USER stack before teh
> :kernel switched to the kernel stack it would have a great simplifying
> :effect on kernel threads support :-) (I know that could lead to traps
> :during saving the context but..)
> [..]
> Julian, you shouldn't worry about userland<->kernel context switch so muc
h.
> The overhead for going into the kernel and coming out again, if all the
> cruft is removed, is *very* tiny -- it's almost like a subroutine call.
I was rather suprised when I found out just how expensive kernel entry was
some time ago.. What I was doing was a reentrant syscall that aquired no
locks and ran about 5 instructions in kernel context.. Anyway, it took
something like 300 times longer to do that (called via int $0x81) than to
do a 'call' to equivalent code in userland. Anyway, with overheads on that
scale, whether we push 5 or 8 or whatever registers in the handler is
almost lost in the noise.
Cheers,
-Peter
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