Re: [PATCH 4/7] KVM: lapic: Allow user to override auto-tuning of timer advancement

[Sean Christopherson <sean.j.christopherson@intel.com>]

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@intel.com> 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@tencent.com>
> > Fixes: 3b8a5df6c4dc6 ("KVM: LAPIC: Tune lapic_timer_advance_ns automatically")
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> 
> 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 :-)

> 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.

> 
> -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
> > 
>