[Simh] vax idle for NetBSD

[Anders Magnusson]

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).

-- R