[Simh] Throttling and this release

[Mark Pizzolato - Info Comm]

On Saturday, July 07, 2012 at 9:33 AM, Gregg Levine wrote:
> On 7 July 2012 07:00, Gregg Levine <gregg.drwho8 at gmail.com> wrote:
> > Hello!
> > Was the throttle function included as part of this release of SIMH? I
> > finally managed to get NetBSD/VAX installed, but it managed to corral
> > the entire workload of the host at 100% of the CPU.
> >
> > I want to throttle it back to about 75%.
> 
> Alternatively if there is a specific pattern which would be best for
> simh to detect idle it should be easy enough to adjust NetBSD's idle
> loop to match...

Hi there,

I'm maintaining the SIMH vax simulator which, for many, has become the replacement for physical vax hardware as more aging hardware just stops working, or folks are merely interested in virtualizing the activities which they've been using vax systems for.

I have recently revisited the simh vax simulator's support for detecting guest OS being in an idle state, and completely reimplemented what had been done in the past.  The prior implementation may have caught the transition into the idle state, but any simulator device's polling could easily take it out of the idle state.  The current implementation reliably detects that a guest OS is IN an idle state not merely the transition to it.

We've got this concept working for all of the "Legacy" operating systems (Legacy meaning old and unchanging).  Meanwhile, the BSD derived operating systems (NetBSD and OpenBSD) still provide vax architecture support and hence are moving targets.  Over time, NetBSD and OpenBSD have internally adjusted what the concept of what/how to idle things as support for multi-core systems and systems with low power states have been implemented.  For the vax architecture, what had been done while the OS was idle in the early BSD days has changed.  

Idle support in the simulator is achieved by detecting some condition which is unique to when a system is actually idling.  For some OSes (i.e. older versions of NetBSD) this involves things like a particular instruction (TSTL), executing in low core system space (8000000-80003FFF), on the interrupt stack, with a 6 byte TSTL instruction with a zero status (this was previously visible in subr.S).  That pattern is no longer correct for probably many versions of NetBSD.  When I trace instructions executed I can't find a unique pattern which is only done while idling.  So, I dug through the source to try and found how things are done now and currently there is no platform specific code in the vax idle path (in kern_idle.c\idle_loop()) .  There is a platform specific "cpu_idle()" macro defined in usr/src/sys/arch/vax/include/cpu.h which expands to nothing in the current code ("#define cpu_idle() do {} while (/*CONSCOND*/0)" ).  To provide a unique pattern which simh can detect, this macro could be changed to:

      #define cpu_idle() do { _splset(splx(1)); } while (/*CONSCOND*/0)

This would give me a unique pattern to detect NetBSD guest OS idling (i.e. setting IPL to 1 (which is otherwise unused in current versions of NetBSD)).  Meanwhile, this code wouldn't hurt anything when run on real vax hardware, and it wouldn't affect any platform independent code (i.e. kern_idle.c).

I've added detection of this pattern when the simulator is configured for NetBSD Idle detection to the codebase of the simh vax simulator.

What would it take for this change, or something like it, be added to the NetBSD source forward?  

Thanks.

-	Mark Pizzolato