[Simh] vax idle for NetBSD

[Anders Magnusson]

Den 2018-12-11 kl. 17:02, skrev 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.
>
No, but it could serve the same purpose, since the opcode is defined.
Then we don't need any more workarounds in simh due to code changes.

-- R