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Subject: Re: [PATCH v4 4/5] KVM: LAPIC: Delay trace advance expire delta
To:     Wanpeng Li <kernellwp@gmail.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Liran Alon <liran.alon@oracle.com>
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 <CANRm+CzFQy4UC9oGxFK8UVVhdtV_LGeF3JcNohpRcgspSqcxwg@mail.gmail.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <024a0c93-f8a3-abe0-85de-fa41babf06a0@redhat.com>
Date:   Mon, 20 May 2019 13:41:23 +0200
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On 20/05/19 13:36, Wanpeng Li wrote:
>> Hmm, yeah, that makes sense.  The location of the tracepoint is a bit
>> weird, but I guess we can add a comment in the code.
> Do you need me to post a new patchset? :)

No problem.  The final patch that I committed is this:

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index c12b090f4fad..f8615872ae64 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1502,27 +1502,27 @@ static inline void __wait_lapic_expire(struct kvm_vcpu *vcpu, u64 guest_cycles)
 }
 
 static inline void adjust_lapic_timer_advance(struct kvm_vcpu *vcpu,
-					      u64 guest_tsc, u64 tsc_deadline)
+					      s64 advance_expire_delta)
 {
 	struct kvm_lapic *apic = vcpu->arch.apic;
 	u32 timer_advance_ns = apic->lapic_timer.timer_advance_ns;
 	u64 ns;
 
 	/* too early */
-	if (guest_tsc < tsc_deadline) {
-		ns = (tsc_deadline - guest_tsc) * 1000000ULL;
+	if (advance_expire_delta < 0) {
+		ns = -advance_expire_delta * 1000000ULL;
 		do_div(ns, vcpu->arch.virtual_tsc_khz);
 		timer_advance_ns -= min((u32)ns,
 			timer_advance_ns / LAPIC_TIMER_ADVANCE_ADJUST_STEP);
 	} else {
 	/* too late */
-		ns = (guest_tsc - tsc_deadline) * 1000000ULL;
+		ns = advance_expire_delta * 1000000ULL;
 		do_div(ns, vcpu->arch.virtual_tsc_khz);
 		timer_advance_ns += min((u32)ns,
 			timer_advance_ns / LAPIC_TIMER_ADVANCE_ADJUST_STEP);
 	}
 
-	if (abs(guest_tsc - tsc_deadline) < LAPIC_TIMER_ADVANCE_ADJUST_DONE)
+	if (abs(advance_expire_delta) < LAPIC_TIMER_ADVANCE_ADJUST_DONE)
 		apic->lapic_timer.timer_advance_adjust_done = true;
 	if (unlikely(timer_advance_ns > 5000)) {
 		timer_advance_ns = 0;
@@ -1545,13 +1545,13 @@ void wait_lapic_expire(struct kvm_vcpu *vcpu)
 	tsc_deadline = apic->lapic_timer.expired_tscdeadline;
 	apic->lapic_timer.expired_tscdeadline = 0;
 	guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
-	trace_kvm_wait_lapic_expire(vcpu->vcpu_id, guest_tsc - tsc_deadline);
+	apic->lapic_timer.advance_expire_delta = guest_tsc - tsc_deadline;
 
 	if (guest_tsc < tsc_deadline)
 		__wait_lapic_expire(vcpu, tsc_deadline - guest_tsc);
 
 	if (unlikely(!apic->lapic_timer.timer_advance_adjust_done))
-		adjust_lapic_timer_advance(vcpu, guest_tsc, tsc_deadline);
+		adjust_lapic_timer_advance(vcpu, apic->lapic_timer.advance_expire_delta);
 }
 
 static void start_sw_tscdeadline(struct kvm_lapic *apic)
diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
index d6d049ba3045..3e72a255543d 100644
--- a/arch/x86/kvm/lapic.h
+++ b/arch/x86/kvm/lapic.h
@@ -32,6 +32,7 @@ struct kvm_timer {
 	u64 tscdeadline;
 	u64 expired_tscdeadline;
 	u32 timer_advance_ns;
+	s64 advance_expire_delta;
 	atomic_t pending;			/* accumulated triggered timers */
 	bool hv_timer_in_use;
 	bool timer_advance_adjust_done;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e7e57de50a3c..35631505421c 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8008,6 +8008,13 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 	++vcpu->stat.exits;
 
 	guest_exit_irqoff();
+	if (lapic_in_kernel(vcpu)) {
+		s64 delta = vcpu->arch.apic->lapic_timer.advance_expire_delta;
+		if (delta != S64_MIN) {
+			trace_kvm_wait_lapic_expire(vcpu->vcpu_id, delta);
+			vcpu->arch.apic->lapic_timer.advance_expire_delta = S64_MIN;
+		}
+	}
 
 	local_irq_enable();
 	preempt_enable();

so that KVM tracks whether wait_lapic_expire was called, and do not
invoke the tracepoint if not.

Thanks,

Paolo