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

Nadav Amit
2019-08-20T04:09:01Z

> On Aug 19, 2019, at 6:56 PM, Sean Christopherson <sean.j.christopherson@intel.com> wrote:
>
> +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.

These tests pass on bare-metal.