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

[Tim Deegan <tim@xxxxxxx>]

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.

Tim.

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