Re: [Xen-devel] [PATCH] x86/HVM: tie RTC emulation mode to enabling of Viridian emulation

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Tue, Jul 2, 2013 at 11:35 AM, Tim Deegan <tim@xxxxxxx> wrote:
> At 11:22 +0100 on 02 Jul (1372764141), Jan Beulich wrote:
>> >>> On 02.07.13 at 11:51, Tim Deegan <tim@xxxxxxx> wrote:
>> > At 10:27 +0100 on 02 Jul (1372760862), Jan Beulich wrote:
>> >> >>> On 02.07.13 at 11:11, Tim Deegan <tim@xxxxxxx> wrote:
>> >> > At 08:02 +0100 on 02 Jul (1372752161), Jan Beulich wrote:
>> >> >> As the mode not conforming to the hardware specification (by allowing
>> >> >> the guest to skip the REG C reads in its interrupt handler) is a
>> >> >> Viridian invention, it seems logical to tie this mode to that extension
>> >> >> being enabled. If the extension is disabled, proper hardware emulation
>> >> >> will be done instead.
>> >> >>
>> >> >> The main thing necessary here is the synchronization of the RTC
>> >> >> emulation code and the setting of the respective flag in hvmloader's
>> >> >> creation of the ACPI WAET table.
>> >> >>
>> >> >> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> >> >
>> >> > Wasn't this going to have its own param, defaulting to off on create and
>> >> > to on on migrate?  I suspect most people just leave the viridian flag on
>> >> > for all domains.
>> >>
>> >> In which case there would be no behavioral difference to what
>> >> we're going to release with 4.3. (That's leaving aside the fact that
>> >> I think people doing so is not the best practice.)
>> >
>> > Why not?  The Viridian interfaces is pretty well essential for running
>> > recent Windows, and shouldn't be harmful for other OSes.
>>
>> Shouldn't. But as we learned it occasionally is - Linux when built
>> without CONFIG_XEN_PVHVM detects the HyperV functionality,
>> and tried using HyperV functionality that Xen doesn't really emulate
>> (see commits 32068f65 ["x86: Hyper-V: register clocksource only if
>> its advertised"] and db34bbb7 ["X86: Add a check to catch Xen
>> emulation of Hyper-V"]).
>
> So the argument is that host admins should already be disabling this
> for _all_ non-windows VMs to work around a bug in some linux kernels,
> and therefore it's OK to hook this vaguely related feature onto it?
> That seems to be below the standard that you'd expect from other
> people.
>
> Anyway, surely we want this bit turned off by default even on Windows,
> to avoid running pointless timers if Windows decides not to use the RTC.

FWIW I agree with this reasoning.  Working around bugs in Linux is a
losing game; and disabling Viridian features that aren't a positive
benefit to Windows seems like a good strategy.

 -George

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