> On 16 Apr 2019, at 19:39, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
>
> On Mon, Apr 15, 2019 at 08:25:48PM +0300, Liran Alon wrote:
>>
>>
>>> On 15 Apr 2019, at 19:32, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
>>>
>>> On Sun, Apr 14, 2019 at 03:15:41PM +0300, Liran Alon wrote:
>>>>
>>>>
>>>>> On 12 Apr 2019, at 23:18, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
>>>>>
>>>>> Calling apic_timer_expired() is a nop when a timer interrupt is already
>>>>> pending, i.e. there's no need to call apic_timer_expired() when there's
>>>>> a pending interrupt and the hv_timer wants to pend its own interrupt.
>>>>> Separate the two flows to make the code more readable and to avoid an
>>>>> unnecessary function call and read to ktimer->pending.
>>>>
>>>> In case timer is not periodic and r==1, atomic_read(&ktimer->pending) is not executed.
>>>>
>>>>>
>>>>> Cc: Wanpeng Li <wanpengli@xxxxxxxxxxx>
>>>>> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
>>>>> ---
>>>>> arch/x86/kvm/lapic.c | 9 ++++++---
>>>>> 1 file changed, 6 insertions(+), 3 deletions(-)
>>>>>
>>>>> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
>>>>> index 1d649a2af04c..f0be6f148a47 100644
>>>>> --- a/arch/x86/kvm/lapic.c
>>>>> +++ b/arch/x86/kvm/lapic.c
>>>>> @@ -1703,9 +1703,12 @@ static bool start_hv_timer(struct kvm_lapic *apic)
>>>>> * the window. For periodic timer, leave the hv timer running for
>>>>> * simplicity, and the deadline will be recomputed on the next vmexit.
>>>>> */
>>>>> - if (!apic_lvtt_period(apic) && (r || atomic_read(&ktimer->pending))) {
>>>>> - if (r)
>>>>> - apic_timer_expired(apic);
>>>>> + if (!apic_lvtt_period(apic) && atomic_read(&ktimer->pending))
>>>>> + return false;
>>>>> +
>>>>> + /* set_hv_timer() returns '1' when the timer has already expired. */
>>>>> + if (r) {
>>>>> + apic_timer_expired(apic);
>>>>> return false;
>>>>> }
>>>>>
>>>>> --
>>>>> 2.21.0
>>>>>
>>>>
>>>> First, I think you should emphasise in commit message that you have actually
>>>> fixed a rare bug here. In case timer is periodic but given
>>>> ktimer->tscdeadline has already expired on host, we should call
>>>> apic_timer_expired().
>>>
>>> Heh, I actually didn't even catch that bug, I was simply cleaning up the
>>> code because I had a hard time following the logic.
>>
>> LOL. So you can put me in the Reported-by tag :P
>
> Actually, thinking about this more, I believe the original behavior was
> correct, if poorly documented. More info below.
>
>>>> In addition, when start_hv_timer() returns false, restart_apic_timer() just
>>>> calls start_sw_timer() which use hrtimer instead of VMX preemption timer.
>>>> Therefore, it seems a bit ineffective to me for start_hv_timer() to return
>>>> false in case ktimer->pending or when ktimer->tscdeadline already expired.
>>>> Shouldn’t we return true in these cases?
>>>
>>> That also seemed weird to me. Again, I had a hell of a time following the
>>> intended logic and didn't want to break anything. AFAICT, the motivation
>>> for calling start_sw_timer() is to cancel the HV timer, and possibly to
>>> ensure start_sw_period() is called when necessary.
>>
>> I think the motivation is that if there is any reason why hardware
>> accelerated timer (i.e. VMX preemption timer), can't be used to emulate the
>> LAPIC timer, then utilise a software hrtimer based implementation instead.
>
> My comment was regarding why start_hv_timer() returns was when the hv_timer
> as already expired.
>
>> This does align with why we return false when (!kvm_x86_ops->set_hv_timer) or
>> (kvm_x86_ops->set_hv_timer() < 0). However, this doesn’t align in case we
>> have a (non-periodic timer and ktimer->pending) OR ktimer->tscdeadline
>> already expired OR (!ktimer->tscdeadline).
>>
>> In fact, note that start_sw_timer() early-exit when non-periodic timer and
>> ktimer->pending… Same is also true for start_sw_tscdeadline() early-exit when
>> (!ktimer->tscdeadline).
>>
>>> But the latter will be
>>> handled by virtue of checking "r" after apic_lvtt_period(), so this?
>>>
>>> if (r) {
>>> apic_timer_expired(apic);
>>> ktimer->hv_timer_in_use = false;
>>> return true;
>>> }
>>
>> I think I will just submit a patch to fix all the above examples I made as
>> this just seems wrong to me. Unless you find something I have missed. :P
>
> When the timer is periodic, we're relying on the timer handler to invoke
> advance_periodic_target_expiration() by way of kvm_lapic_expired_hv_timer().
> That's why the original code only checks @r if apic_lvtt_period()==false,
> i.e. to actually trigger a VMX preemption timer VM-Exit. Note that the
> return from set_hv_timer() is essentially a hint, e.g. VMX is perfectly
> fine programming a preemption timer with a value of zero.
Yes I understood that already.
I don’t think it contradicts the fact that the checks I mentioned above should be moved out of start_hv_timer().
>
> I think Paolo's suggestion of moving the logic up into restart_apic_timer()
> is the way to go as it reduces the multiplexing down on start_hv_timer()'s
> return value.
>
Yes I agree. I plan to do so.
-Liran
