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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2013-04-29 at 12:27 +0100, Jan Beulich wrote:
> >>> On 29.04.13 at 13:07, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> > 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...
> 
> Considering that the RTC register accesses are indirect, a REG_C
> read is actually a port 70 write and a port 71 read. And then they
> repeat this at least twice (and more times if necessary until they
> find bit 7 of the value read to be clear).
> 
> So minimally 4 exits for one RTC interrupt.

Thanks, I knew there'd be something ;-)

Ian.



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