[Simh] vax idle for NetBSD

[David Brownlee]

On Sun, 9 Dec 2018 at 09:42, Mark Pizzolato <Mark at infocomm.com> wrote:
>
> Well, when I followed the directions you pointed out, I did see what you
> had reported.  A little deeper observation strongly suggested that the
> OS execution flow that happens on NetBSD/VAX during idling had changed
> from how things were working when I had worked through how to get things
> well behaved idling under simh.
>
> I haven't yet had the time to look closely at the OS code to determine
> what exactly was happening.  If you're seeing things OK now, maybe, there
> is some sort of garbage collection activity going on, which runs for a while
> and then somehow eventually stabilizes to the known idle execution
> pattern.
>
> NetBSD is the fundamental exception for operating systems running under
> simh due to the fact that it is still an actively developed and evolving
> operating system.  As a result, the idle detection pattern will always have
> to evolve along with the evolving OS.

That also cuts both ways (but in a good way :) - because it is still
under active development it is reasonable to tweak the code to work
better under simh (other ports like amd64, aarch64 and mips are
adjusted all the time to work better under qemu), so if there is a
better way for NetBSD to tag the idle loop, lets make it so :)

For anyone interested the current logic has:

        int ipl = mfpr(PR_IPL);
        mtpr(1, PR_IPL);
        mtpr(ipl, PR_IPL);

Which looks to have been there since 2012 - see
https://nxr.netbsd.org/xref/src/sys/arch/vax/include/cpu.h?r=1.102#162

Thanks

David