linux-kernel - Re: [PATCH] KVM: LAPIC: Reset timer_advance

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Date:   Wed, 2 Sep 2020 14:23:28 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH] KVM: LAPIC: Reset timer_advance_ns if timer mode switch

On Fri, Aug 28, 2020 at 09:35:08AM +0800, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> per-vCPU timer_advance_ns should be set to 0 if timer mode is not tscdeadline 
> otherwise we waste cpu cycles in the function lapic_timer_int_injected(), 
> especially on AMD platform which doesn't support tscdeadline mode. We can 
> reset timer_advance_ns to the initial value if switch back to tscdealine 
> timer mode.
> 
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>  arch/x86/kvm/lapic.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 654649b..abc296d 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1499,10 +1499,16 @@ static void apic_update_lvtt(struct kvm_lapic *apic)
>  			kvm_lapic_set_reg(apic, APIC_TMICT, 0);
>  			apic->lapic_timer.period = 0;
>  			apic->lapic_timer.tscdeadline = 0;
> +			if (timer_mode == APIC_LVT_TIMER_TSCDEADLINE &&
> +				lapic_timer_advance_dynamic)

Bad indentation.

> +				apic->lapic_timer.timer_advance_ns = LAPIC_TIMER_ADVANCE_NS_INIT;

Redoing the tuning seems odd.  Doubt it will matter, but it feels weird to
have to retune the advancement just because the guest toggled between modes.

Rather than clear timer_advance_ns, can we simply move the check against
apic->lapic_timer.expired_tscdeadline much earlier?  I think that would
solve this performance hiccup, and IMO would be a logical change in any
case.  E.g. with some refactoring to avoid more duplication between VMX and
SVM:

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 35cca2e0c8026..54222f0071547 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1571,12 +1571,12 @@ static inline void adjust_lapic_timer_advance(struct kvm_vcpu *vcpu,
        apic->lapic_timer.timer_advance_ns = timer_advance_ns;
 }

-static void __kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
+void __kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
 {
        struct kvm_lapic *apic = vcpu->arch.apic;
        u64 guest_tsc, tsc_deadline;

-       if (apic->lapic_timer.expired_tscdeadline == 0)
+       if (!lapic_timer_int_injected(vcpu))
                return;

        tsc_deadline = apic->lapic_timer.expired_tscdeadline;
@@ -1590,13 +1590,7 @@ static void __kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
        if (lapic_timer_advance_dynamic)
                adjust_lapic_timer_advance(vcpu, apic->lapic_timer.advance_expire_delta);
 }
-
-void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
-{
-       if (lapic_timer_int_injected(vcpu))
-               __kvm_wait_lapic_expire(vcpu);
-}
-EXPORT_SYMBOL_GPL(kvm_wait_lapic_expire);
+EXPORT_SYMBOL_GPL(__kvm_wait_lapic_expire);

 static void kvm_apic_inject_pending_timer_irqs(struct kvm_lapic *apic)
 {
diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
index 754f29beb83e3..64be9d751196a 100644
--- a/arch/x86/kvm/lapic.h
+++ b/arch/x86/kvm/lapic.h
@@ -236,7 +236,14 @@ static inline int kvm_lapic_latched_init(struct kvm_vcpu *vcpu)

 bool kvm_apic_pending_eoi(struct kvm_vcpu *vcpu, int vector);

-void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu);
+void __kvm_wait_lapic_expire(struct kvm_vcpu *vcpu);
+static inline void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
+{
+       if (lapic_in_kernel(vcpu) &&
+           vcpu->arch.apic->lapic_timer.expired_tscdeadline &&
+           vcpu->arch.apic->lapic_timer.timer_advance_ns)
+               __kvm_wait_lapic_expire(vcpu);
+}

 void kvm_bitmap_or_dest_vcpus(struct kvm *kvm, struct kvm_lapic_irq *irq,
                              unsigned long *vcpu_bitmap);
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index eee7edcbe7491..dfe505a7304a3 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3456,9 +3456,7 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu)
        clgi();
        kvm_load_guest_xsave_state(vcpu);

-       if (lapic_in_kernel(vcpu) &&
-               vcpu->arch.apic->lapic_timer.timer_advance_ns)
-               kvm_wait_lapic_expire(vcpu);
+       kvm_wait_lapic_expire(vcpu);

        /*
         * If this vCPU has touched SPEC_CTRL, restore the guest's value if


>  		}
>  		apic->lapic_timer.timer_mode = timer_mode;
>  		limit_periodic_timer_frequency(apic);
>  	}
> +	if (timer_mode != APIC_LVT_TIMER_TSCDEADLINE &&
> +		lapic_timer_advance_dynamic)

Bad indentation.

> +		apic->lapic_timer.timer_advance_ns = 0;
>  }
>  
>  /*
> -- 
> 2.7.4
>