[Simh] vax idle for NetBSD

[Paul Koning]

> On Dec 11, 2018, at 10:47 AM, Anders Magnusson <ragge at ludd.ltu.se> wrote:
> 
> Den 2018-12-11 kl. 11:11, skrev 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
>> 
> Hm, the vax actually got a WAIT instruction in later versions of the hardware.
> It could be an idea to use it (and add code to catch it if not implemented).

The VAX architecture standard (DEC Std 032, from Bitsavers) mentions that as a virtual machine instruction that's trapped and handled by the hypervisor.  It doesn't sound like a real instruction, the way it is on a PDP-11.

	paul