Re: [Xen-devel] [Patch] x86/HVM: Fix RTC interrupt modelling

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 02/10/2014 04:34 PM, Jan Beulich wrote:
> >    * The results from XenRT suggest that the new emulation is better than the
> >      old.
> "Better" in the sense of the limited set of uses of the virtual hardware
> by whatever selection of guest OSes is being run there. But very
> likely not "better" in the sense on matching up with how the respective
> hardware specification would require it to behave.
The context of the above sentence was in a justification for including 
it in 4.4.  Obviously "occasionally gets stuck during boot" is a pretty 
bad bug that we'd like to see fix.  But given the tricky nature of this 
whole area, there's a risk that this will cause regressions in *other* 
situations or operating systems.  What I understand Andy to be saying is 
that with the patch, the RTC appears to cause less problems than without it.
What your analysis is missing, Andy, is what the effects might be if 
there were a bug.  Obviously other guests might hang during boot; but 
what else?  Might they hang at some point much later, perhaps when being 
pounded with interrupts due to heavy network traffic? Might the clock 
begin to drift or jump around?  Would the XenRT testing catch that if it 
happened?  And, would those potential bugs be worse than what we have now?
There's a reason for trying to go through the whole exercise, 
particularly in bugs like this.  I do have a lot of faith in our 
intuition to consider hundreds of individual factors and come up with a 
reasonably good judgement of the probabilities -- but only if it is 
guided properly.  We are all very prone to only consider the things we 
happen to be thinking about, and to completely ignore all the things we 
don't happen to be looking at.  My own temptation, looking at this bug, 
is to say, "Random hangs during boot, yeah, that's pretty bad; we should 
take it."  But then I'm only looking at the positives of the patch: I'm 
not really making a balance of the positives versus the negatives.
The goal of going through the "worst-case-scenario" exercise is to bring 
to our minds the potential outcomes we are prone to ignore. Only then 
can we reasonably trust our intuition to make a properly informed judgement.
 -George

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