Re: [Xen-devel] Bug: Windows 2003 fails to install on xen-unstable tip

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 25.04.13 at 18:34, Tim Deegan <tim@xxxxxxx> wrote:
> At 17:02 +0100 on 25 Apr (1366909369), Jan Beulich wrote:
>> With that fixed and the mentioned code block removed, things
>> work as I had expected. But the logging that I added in the
>> course of all this shows that it really juts happens to work,
>> I can't really explain why (other than the myriads of superfluous
>> interrupts attempted to be injected into the guest keeping the
>> VM alive). In particular, almost none of the injected IRQs
>> actually reach their handler (there are only very few REG_C
>> reads), but the handler also doesn't do anything really
>> interesting (i.e. we don't actually need the handler to execute,
>> we just need to keep a flow of interrupts going into the VM).
> 
> Really?  Does injecting spurious interrupts work too?  Presumably the
> handler does _something_.

I have no sign of the handler being reached at all, except in
very few (i.e. exceptional) cases, usually the first one to three
instances. But I still can't tell why that is. After having monitored
the VM_ENTRY_INTR_INFO to be correct, I'm now planning to
watch the IRR and ISR bits, as somehow they ought to get
cleared.

>> Yet I did verify that the correct values get actually written
>> through vmx_inject_extint()/__vmx_inject_exception().
>> 
>> So at this point I'm of the opinion that the RTC changes really
>> just exposed a completely different issue, and I'm of the opinion
>> that this is what needs fixing, not papering over it by reverting
>> the RTC stuff.
> 
> On the contrary, I think this shows that the RTC/vpt/guest interactions
> are so badly understood as to merit backing the whole lot out for 4.3.
> This has been dragging on for weeks now, and AFAICS it's going to need a
> serious overhaul, plus someone manually testing all the OSes that are
> likely to be affected (i.e. the crufty old ones).

After more thinking about this overnight I tend to agree, albeit
in the milder form I always advocated for (i.e. back out just the
RTC/VPT interaction change). Of course that implies that we'll
continue to run at least w2k3 without understanding why it works
at all, which from a support perspective is _really_ bad.

Jan


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