Re: [Xen-devel] More RTC issues with Win2k3

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 03.09.13 at 15:37, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> Since updating to Xen 4.3 in XenServer trunk, we have been seeing these
> issues a handful of times on each nightly test.  We are currently
> running stable-4.3 with no relevant patches in this area.
> 
> The issue exists solely with Win2k3 Enterprise Edition SP2 64bit, and
> occurs sporadically in a simple reboot loop of the VM.
> 
> The VM sits at 100% CPU performing:
> 
> d98v0 vmexit exit_reason IO_INSTRUCTION eip fffff80000814176
> d98v0 io write port 70 val c
> d98v0 vmentry cycles 1944
> d98v0 vmexit exit_reason IO_INSTRUCTION eip fffff80000814177
> d98v0 io read port 71 val c0
> d98v0 vmentry cycles 3120
> 
> I am attempting to narrow down which conditions cause it to fall into
> this loop, but am really picking blindly at factors.  One possibility is
> the domain time offset being set to -24 seconds, but I cant immediately
> see why that would have an effect.

So based on my code inspection last time I'd expect this to be a
tight loop in the RTC interrupt handler, waiting for REG_C to read
as zero.

Which raises two questions: Does that specific version of Windows
not honor the WAET flags saying that REG_C reads are unnecessary?
Or does this only occur during very early boot (where iirc a first,
temporary RTC interrupt handler gets installed for a very brief period
of time that doesn't pay attention to the WAET flag)?

> I have attached xen-hvmctx from the affected domain, and do have one
> example of a VM in this loop so can poke for other state, if there are
> any sensible suggestions

The REG_A value says 64Hz for the periodic interrupt if I'm not
mistaken, so RTC_PF getting re-set between two iterations would
first of all hint at a significantly overcommitted system (such that
no two iterations of the loop can complete within 1/64 second).

Jan


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