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 :-) > 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 > > >