linux-kernel - [RFC PATCH v3 12/21] KVM: x86: Remove implicit rdtsc() from kvm_compute_l1_tsc

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Date: Wed, 22 May 2024 01:17:07 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: kvm@...r.kernel.org
Cc: Paolo Bonzini <pbonzini@...hat.com>,
	Jonathan Corbet <corbet@....net>,
	Sean Christopherson <seanjc@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Paul Durrant <paul@....org>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>,
	Mel Gorman <mgorman@...e.de>,
	Daniel Bristot de Oliveira <bristot@...hat.com>,
	Valentin Schneider <vschneid@...hat.com>,
	Shuah Khan <shuah@...nel.org>,
	linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	jalliste@...zon.co.uk,
	sveith@...zon.de,
	zide.chen@...el.com,
	Dongli Zhang <dongli.zhang@...cle.com>,
	Chenyi Qiang <chenyi.qiang@...el.com>
Subject: [RFC PATCH v3 12/21] KVM: x86: Remove implicit rdtsc() from kvm_compute_l1_tsc_offset()

From: David Woodhouse <dwmw@...zon.co.uk>

Let the callers pass the host TSC value in as an explicit parameter.

This leaves some fairly obviously stupid code, which using this function
to compare the guest TSC at some *other* time, with the newly-minted TSC
value from rdtsc(). Unless it's being used to measure *elapsed* time,
that isn't very sensible.

In this case, "obviously stupid" is an improvement over being non-obviously
so.

No functional change intended.

Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
---
 arch/x86/kvm/x86.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ef3cd6113037..ea59694d712a 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2601,11 +2601,12 @@ u64 kvm_scale_tsc(u64 tsc, u64 ratio)
 	return _tsc;
 }
 
-static u64 kvm_compute_l1_tsc_offset(struct kvm_vcpu *vcpu, u64 target_tsc)
+static u64 kvm_compute_l1_tsc_offset(struct kvm_vcpu *vcpu, u64 host_tsc,
+				     u64 target_tsc)
 {
 	u64 tsc;
 
-	tsc = kvm_scale_tsc(rdtsc(), vcpu->arch.l1_tsc_scaling_ratio);
+	tsc = kvm_scale_tsc(host_tsc, vcpu->arch.l1_tsc_scaling_ratio);
 
 	return target_tsc - tsc;
 }
@@ -2758,7 +2759,7 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 *user_value)
 	bool synchronizing = false;
 
 	raw_spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags);
-	offset = kvm_compute_l1_tsc_offset(vcpu, data);
+	offset = kvm_compute_l1_tsc_offset(vcpu, rdtsc(), data);
 	ns = get_kvmclock_base_ns();
 	elapsed = ns - kvm->arch.last_tsc_nsec;
 
@@ -2809,7 +2810,7 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 *user_value)
 		} else {
 			u64 delta = nsec_to_cycles(vcpu, elapsed);
 			data += delta;
-			offset = kvm_compute_l1_tsc_offset(vcpu, data);
+			offset = kvm_compute_l1_tsc_offset(vcpu, rdtsc(), data);
 		}
 		matched = true;
 	}
@@ -4024,7 +4025,8 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		if (msr_info->host_initiated) {
 			kvm_synchronize_tsc(vcpu, &data);
 		} else {
-			u64 adj = kvm_compute_l1_tsc_offset(vcpu, data) - vcpu->arch.l1_tsc_offset;
+			u64 adj = kvm_compute_l1_tsc_offset(vcpu, rdtsc(), data) -
+				vcpu->arch.l1_tsc_offset;
 			adjust_tsc_offset_guest(vcpu, adj);
 			vcpu->arch.ia32_tsc_adjust_msr += adj;
 		}
@@ -5098,7 +5100,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 			mark_tsc_unstable("KVM discovered backwards TSC");
 
 		if (kvm_check_tsc_unstable()) {
-			u64 offset = kvm_compute_l1_tsc_offset(vcpu,
+			u64 offset = kvm_compute_l1_tsc_offset(vcpu, rdtsc(),
 						vcpu->arch.last_guest_tsc);
 			kvm_vcpu_write_tsc_offset(vcpu, offset);
 			vcpu->arch.tsc_catchup = 1;
-- 
2.44.0