> 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
>
>> 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.
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
-Liran
