Re: [Qemu-devel] [PATCH v5] i386: Introduce ARAT CPU feature

[Wanpeng Li]

On 6/22/15 1:38 AM, Jan Kiszka wrote:
> On 2015-06-18 22:21, Eduardo Habkost wrote:
> > On Sun, Jun 07, 2015 at 11:15:08AM +0200, Jan Kiszka wrote:
> > > From: Jan Kiszka <address@hidden>
> > >
> > > ARAT signals that the APIC timer does not stop in power saving states.
> > > As our APICs are emulated, it's fine to expose this feature to guests,
> > > at least when asking for KVM host features or with CPU types that
> > > include the flag. The exact model number that introduced the feature is
> > > not known, but reports can be found that it's at least available since
> > > Sandy Bridge.
> > >
> > > Signed-off-by: Jan Kiszka <address@hidden>
> > The code looks good now, but: what are the real consequences of
> > enabling/disabling the flag? What exactly guests use it for?
> >
> > Isn't this going to make guests have additional expectations about the
> > APIC timer that may be broken when live-migrating or pausing the VM?
> ARAT only refers to stopping of the timer in certain power states (which
> we do not even emulate IIRC). In that case, the OS is under risk of
> sleeping forever, thus need to look for a different wakeup source.

HPET will always be the default broadcast event device I think.

Regards,
Wanpeng Li

> Live-migration or VM pausing are external effects on all timers of the
> guest, not only the APIC. However, none of them cause a wakeup miss -
> provided the host decides to resume the guest eventually.
>
> Jan