Re: [PATCH] KVM: lapic: restart counter on change to periodic mode

Sean Christopherson
2019-08-20T01:56:44Z

+Cc Nadav

On Mon, Aug 19, 2019 at 06:07:01PM -0700, Matt Delco wrote:
> On Mon, Aug 19, 2019 at 5:37 PM Sean Christopherson <
> sean.j.christopherson@intel.com> wrote:
>
> > On Tue, Aug 20, 2019 at 01:42:37AM +0200, Paolo Bonzini wrote:
> > > On 20/08/19 01:04, Matt delco wrote:
> > > > From: Matt Delco <delco@google.com>
> > > >
> > > > Time seems to eventually stop in a Windows VM when using Skype.
> > > > Instrumentation shows that the OS is frequently switching the APIC
> > > > timer between one-shot and periodic mode. The OS is typically writing
> > > > to both LVTT and TMICT. When time stops the sequence observed is that
> > > > the APIC was in one-shot mode, the timer expired, and the OS writes to
> > > > LVTT (but not TMICT) to change to periodic mode. No future timer
> > events
> > > > are received by the OS since the timer is only re-armed on TMICT
> > writes.
> > > >
> > > > With this change time continues to advance in the VM. TBD if physical
> > > > hardware will reset the current count if/when the mode is changed to
> > > > period and the current count is zero.
> > > >
> > > > Signed-off-by: Matt Delco <delco@google.com>
> > > > ---
> > > > arch/x86/kvm/lapic.c | 9 +++++++--
> > > > 1 file changed, 7 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > > > index 685d17c11461..fddd810eeca5 100644
> > > > --- a/arch/x86/kvm/lapic.c
> > > > +++ b/arch/x86/kvm/lapic.c
> > > > @@ -1935,14 +1935,19 @@ int kvm_lapic_reg_write(struct kvm_lapic
> > *apic, u32 reg, u32 val)
> > > >
> > > > break;
> > > >
> > > > - case APIC_LVTT:
> > > > + case APIC_LVTT: {
> > > > + u32 timer_mode = apic->lapic_timer.timer_mode;
> > > > if (!kvm_apic_sw_enabled(apic))
> > > > val |= APIC_LVT_MASKED;
> > > > val &= (apic_lvt_mask[0] |
> > apic->lapic_timer.timer_mode_mask);
> > > > kvm_lapic_set_reg(apic, APIC_LVTT, val);
> > > > apic_update_lvtt(apic);
> > > > + if (timer_mode == APIC_LVT_TIMER_ONESHOT &&
> > > > + apic_lvtt_period(apic) &&
> > > > + !hrtimer_active(&apic->lapic_timer.timer))
> > > > + start_apic_timer(apic);
> > >
> > > Still, this needs some more explanation. Can you cover this, as well as
> > > the oneshot->periodic transition, in kvm-unit-tests' x86/apic.c
> > > testcase? Then we could try running it on bare metal and see what
> > happens.
> >
>
> I looked at apic.c and test_apic_change_mode() might already be testing
> this. It sets oneshot & TMICT, waits for the current value to get
> half-way, changes the mode to periodic, and then tries to test that the
> value wraps back to the upper half. It then waits again for the half-way
> point, changes the mode back to oneshot, and waits for zero. After
> reaching zero it does:
>
> /* now tmcct == 0 and tmict != 0 */
> apic_change_mode(APIC_LVT_TIMER_PERIODIC);
> report("TMCCT should stay at zero", !apic_read(APIC_TMCCT));
>
> which seems to be testing that oneshot->periodic won't reset the timer if
> it's already zero. A possible caveat is there's hardly any delay between
> the mode change and the timer read. Emulated hardware will react
> instantaneously (at least as seen from within the VM), but hardware might
> need more time to react (though offhand I'd expect HW to be fast enough for
> this particular timer).
>
> So, it looks like the code might already be ready to run on physical
> hardware, and if it has (or does already as part of a regular test), then
> that does raise some doubt on what's the appropriate code change to make
> this work.

Nadav has been running tests on bare metal, maybe he can weigh in on
whether or not test_apic_change_mode() passes on bare metal.