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

>>> 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.


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