> On 16 Apr 2019, at 20:48, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
>
> Add a new KVM param, lapic_timer_advance_autotune, to allow userspace
> to disable auto-tuning the timer advancement, e.g. to manually tune the
> delay.
>
> Note, this does not completely restore the original behavior of
> lapic_timer_advance_ns. Prior to tracking the advancement per vCPU,
> which is necessary to support autotuning, userspace could adjust
> lapic_timer_advance_ns for *running* vCPU. With per-vCPU tracking, the
> module params are snapshotted at vCPU creation, i.e. applying a new
> advancement effectively requires restarting a VM.
>
> Dynamically updating a running vCPU is possible, e.g. a helper could be
> added to retrieve the desired delay, choosing between the global module
> param and the per-VCPU value depending on whether or not auto-tuning is
> (globally) enabled, but introduces a great deal of complexity. The
> wrapper itself is not complex, but understanding and documenting the
> effects of dynamically toggling auto-tuning and/or adjusting the timer
> advancement is nigh impossible since the behavior would be dependent on
> KVM's implementation as well as compiler optimizations. In other words,
> providing stable behavior would require extremely careful consideration
> now and in the future.
>
> Given that the expected use of a manually-tuned timer advancement is to
> "tune once, run many", use the vastly simpler approach of recognizing
> changes to the module params only when creating a new vCPU.
>
> Fixes: 3b8a5df6c4dc6 ("KVM: LAPIC: Tune lapic_timer_advance_ns automatically")
> Cc: Liran Alon <liran.alon@xxxxxxxxxx>
> Cc: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Reviewed-by: Liran Alon <liran.alon@xxxxxxxxxx>
> ---
> arch/x86/kvm/lapic.c | 4 +++-
> arch/x86/kvm/lapic.h | 3 ++-
> arch/x86/kvm/x86.c | 6 +++++-
> 3 files changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 3cd62edf8084..37d3489f68c9 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2263,7 +2263,8 @@ static enum hrtimer_restart apic_timer_fn(struct hrtimer *data)
> return HRTIMER_NORESTART;
> }
>
> -int kvm_create_lapic(struct kvm_vcpu *vcpu, u32 timer_advance_ns)
> +int kvm_create_lapic(struct kvm_vcpu *vcpu, u32 timer_advance_ns,
> + bool timer_advance_autotune)
> {
> struct kvm_lapic *apic;
>
> @@ -2288,6 +2289,7 @@ 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;
> + apic->lapic_timer.timer_advance_adjust_done = !timer_advance_autotune;
>
> /*
> * 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..7fa1aed02c14 100644
> --- a/arch/x86/kvm/lapic.h
> +++ b/arch/x86/kvm/lapic.h
> @@ -64,7 +64,8 @@ struct kvm_lapic {
>
> struct dest_map;
>
> -int kvm_create_lapic(struct kvm_vcpu *vcpu, u32 timer_advance_ns);
> +int kvm_create_lapic(struct kvm_vcpu *vcpu, u32 timer_advance_ns,
> + bool timer_advance_autotune);
> void kvm_free_lapic(struct kvm_vcpu *vcpu);
>
> int kvm_apic_has_interrupt(struct kvm_vcpu *vcpu);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index b303a21a2bc2..60d80e354f62 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -140,6 +140,9 @@ module_param(tsc_tolerance_ppm, uint, S_IRUGO | S_IWUSR);
> static u32 __read_mostly lapic_timer_advance_ns = 1000;
> module_param(lapic_timer_advance_ns, uint, S_IRUGO | S_IWUSR);
>
> +static bool __read_mostly lapic_timer_advance_autotune = true;
> +module_param(lapic_timer_advance_autotune, bool, S_IRUGO | S_IWUSR);
> +
> static bool __read_mostly vector_hashing = true;
> module_param(vector_hashing, bool, S_IRUGO);
>
> @@ -9078,7 +9081,8 @@ int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)
>
> if (irqchip_in_kernel(vcpu->kvm)) {
> vcpu->arch.apicv_active = kvm_x86_ops->get_enable_apicv(vcpu);
> - r = kvm_create_lapic(vcpu, lapic_timer_advance_ns);
> + r = kvm_create_lapic(vcpu, lapic_timer_advance_ns,
> + lapic_timer_advance_autotune);
> if (r < 0)
> goto fail_mmu_destroy;
> } else
> --
> 2.21.0
>
