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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2013-04-29 at 11:29 +0100, Tim Deegan wrote:
> > > >> Now it is obvious that the combination of that flag and proper
> > > >> RTC emulation can't work together,
> > > >
> > > > So does the flag actually require *improper* behaviour from the
> > RTC
> > > > (emulated or otherwise)?
> 
> Specifically emulated: that's what the 'E' in WAET stands for.  This
> flag is to tell Windows that it needn't ack every RTC interrupt with an
> IO read, so avoiding a bunch of VMEXITs (yay!).

Ah, so this is actually a viridian thing? HVM domains do have a viridian
mode already, can we (sanely) gate on that?

And knowing what E stands for now I can now read "GOOD" as "good, if you
are virtualising", which makes more sense.

Don't you get an equivalent number of VMEXITs from the unnecessary
interrupt injections? I'm clearly missing something...

>   But clearly you can't
> have that behaviour and also Jan's change to skip all the interrupts
> entirely if the guest's not using them.

Yes.

> My inclination is:
> 
> - for 4.3, revert.  Even if we keep the various emulation changes, we
>   should go back to having the VPT code inject the interrupts, and
>   drop the idea of suppressing the timer when the interrupt's unused. 

Should perhaps investigate the impact of not setting that specific bit?

> - For 4.4, make this bit optional and turn on the RTC suppression
>   once the VPT interactions are fixed, and only when this bit is clear.
>   The bit should be set off by default for new creation, but on by
>   default on restore/migrate if unspecified by the sender, so that 
>   VMs currently relying on it don't break.
> 
> Tim.



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