> 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.
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?
-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
>
