> On 15 Apr 2019, at 19:23, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
>
> On Sun, Apr 14, 2019 at 02:35:39PM +0300, Liran Alon wrote:
>>
>>
>>> On 12 Apr 2019, at 23:18, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
>>>
>>> Disable auto-tuning the timer advancement if the user specifies an
>>> explicit value via the module param. Aside from the obvious override
>>> capability, this also allows the KVM admin to set the advancement
>>> beyond the internally-capped max of 5000ns.
>>>
>>> Cc: Wanpeng Li <wanpengli@xxxxxxxxxxx>
>>> Fixes: 3b8a5df6c4dc6 ("KVM: LAPIC: Tune lapic_timer_advance_ns automatically")
>>> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
>>
>> I agree we should allow admin the ability to disable the auto-tuning.
>> However, I think we should keep the semantic of lapic_timer_advance_ns value.
>> Whether this value is used as initial-value for auto-tuning or is auto-tuning
>> disabled is a different knob for admin in my opinion. Therefore, I think we
>> should just add another module parameter which basically set the initial
>> value for apic->lapic_timer.timer_advance_adjust_done.
>
> I waffled on whether to add a param or do the implicit override. I opted
> for the implicit behavior primarily because I think most people would
> expect that setting lapic_timer_advance_ns to some specific value would
> fix the advancement at said value, as opposed to acting as a hint to the
> autotuning logic. In other words, I again don't have a strong opinion :-)
Let’s wait for Paolo’s opinion on this then :)
BTW, a nice side-effect of my proposal is that also the current value used by KVM module at runtime (after auto-tuning)
can be viewed by printing current value of lapic_timer_advance_ns module_param.
>
>> One could also wonder if it makes sense that whether auto-tuning will be
>> enabled or not varies between VMs and should not be a global variable of KVM
>> module?
>
> I have no opinion on this one as I don't have any direct visibility into
> how the auto-tuning will be used in real world deployments.
I will wait for Paolo’s & Wanpeng’s opinion on this as-well.
-Liran
>
>>
>> -Liran
>>
>>> ---
>>> arch/x86/kvm/lapic.c | 2 ++
>>> arch/x86/kvm/lapic.h | 2 ++
>>> arch/x86/kvm/x86.c | 2 +-
>>> 3 files changed, 5 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
>>> index 70a0acd98e9e..c43cd26f040b 100644
>>> --- a/arch/x86/kvm/lapic.c
>>> +++ b/arch/x86/kvm/lapic.c
>>> @@ -2286,6 +2286,8 @@ int kvm_create_lapic(struct kvm_vcpu *vcpu, u32 timer_advance_ns)
>>> HRTIMER_MODE_ABS_PINNED);
>>> apic->lapic_timer.timer.function = apic_timer_fn;
>>> apic->lapic_timer.timer_advance_ns = timer_advance_ns;
>>> + if (timer_advance_ns != LAPIC_TIMER_ADVANCE_DEFAULT_NS)
>>> + apic->lapic_timer.timer_advance_adjust_done = true;
>>>
>>> /*
>>> * APIC is created enabled. This will prevent kvm_lapic_set_base from
>>> diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
>>> index 3e97f8a68967..c7233629c05b 100644
>>> --- a/arch/x86/kvm/lapic.h
>>> +++ b/arch/x86/kvm/lapic.h
>>> @@ -16,6 +16,8 @@
>>> #define APIC_BUS_CYCLE_NS 1
>>> #define APIC_BUS_FREQUENCY (1000000000ULL / APIC_BUS_CYCLE_NS)
>>>
>>> +#define LAPIC_TIMER_ADVANCE_DEFAULT_NS 1000
>>> +
>>> enum lapic_mode {
>>> LAPIC_MODE_DISABLED = 0,
>>> LAPIC_MODE_INVALID = X2APIC_ENABLE,
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index b303a21a2bc2..709a8bf5ae0e 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -137,7 +137,7 @@ static u32 __read_mostly tsc_tolerance_ppm = 250;
>>> module_param(tsc_tolerance_ppm, uint, S_IRUGO | S_IWUSR);
>>>
>>> /* lapic timer advance (tscdeadline mode only) in nanoseconds */
>>> -static u32 __read_mostly lapic_timer_advance_ns = 1000;
>>> +static u32 __read_mostly lapic_timer_advance_ns = LAPIC_TIMER_ADVANCE_DEFAULT_NS;
>>> module_param(lapic_timer_advance_ns, uint, S_IRUGO | S_IWUSR);
>>>
>>> static bool __read_mostly vector_hashing = true;
>>> --
>>> 2.21.0
>>>
>>
