[pups] Stray Interupts

Fred N. van Kempen Fred.van.Kempen at microwalt.nl
Mon Feb 26 10:39:25 AEST 2001


Robin Birch wrote:

> > This means that it won't be simple to fix!!!!!!!!
Sure, fix the source :)

> > It could mean that running Ultrix on a simulator isn't on 
> without some work on the emulators themselves. 
I have been running Ultrix-11 V3.1 on Ersatz-11 without any of that. It
_could_ be linked to interrupt latency issues- Ultrix tells controller
to do something (e.g., three commands to read a sector).  Controller does
as told, generating an interrupt for each of the requests saying its ready.
However, because of latency, only ints 1 and 3 actually get delivered within
the expected timeframe (can happen).

Usually (from my experience with writing Unix kernel drivers), this is not
a problem, because the "message" from (in this case) int2 will be picked up
when we start to service int3, which we _did_ see.  So, even though we didnt
get int2, we were fine.

Now... emulator wakes up again, goes "oi, i messed up, better go send that
int now" and sends the int.  The driver no longer _awaits_ an interrupt
we cleared the AttentionNeeded flags when servicing int3), so... we get the
"stray int" message.

If this logic is correct, it will get worse when loading the host system
heavily, so latency will occur more often.  On a very fast box (like my quad
P3/850 Linux box) it should hardly occur.


Fred (hacking on the V3.1 source to not fuck up TCP/IP on the 11/23 ..)

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