[PATCH v8 17/17] KVM: x86: Remove implicit rdtsc() from kvm_compute_l1_tsc_offset()

[Sean Christopherson <seanjc@google.com>]

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

Let the callers pass the host TSC value in as an explicit parameter.
This leaves some fairly obviously stupid code, which is 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@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/msrs.c |  3 ++-
 arch/x86/kvm/x86.c  | 11 ++++++-----
 arch/x86/kvm/x86.h  |  3 ++-
 3 files changed, 10 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/msrs.c b/arch/x86/kvm/msrs.c
index 66fa7140d65d..22ceb39d41f6 100644
--- a/arch/x86/kvm/msrs.c
+++ b/arch/x86/kvm/msrs.c
@@ -1631,7 +1631,8 @@ 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 if (!vcpu->arch.guest_tsc_protected) {
-			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;
 		}
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 02c389523f52..cbe4fd3457b4 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1153,11 +1153,12 @@ u64 kvm_scale_tsc(u64 tsc, u64 ratio)
 	return _tsc;
 }
 
-u64 kvm_compute_l1_tsc_offset(struct kvm_vcpu *vcpu, u64 target_tsc)
+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;
 }
@@ -1319,7 +1320,7 @@ 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;
 
@@ -1368,7 +1369,7 @@ 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;
 	}
@@ -2625,7 +2626,7 @@ 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);
 			if (!vcpu->arch.guest_tsc_protected)
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index 0f5919b092e4..1d9a66048b01 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -335,7 +335,8 @@ u64 kvm_scale_tsc(u64 tsc, u64 ratio);
 u64 kvm_read_l1_tsc(struct kvm_vcpu *vcpu, u64 host_tsc);
 u64 kvm_calc_nested_tsc_offset(u64 l1_offset, u64 l2_offset, u64 l2_multiplier);
 u64 kvm_calc_nested_tsc_multiplier(u64 l1_multiplier, u64 l2_multiplier);
-u64 kvm_compute_l1_tsc_offset(struct kvm_vcpu *vcpu, u64 target_tsc);
+u64 kvm_compute_l1_tsc_offset(struct kvm_vcpu *vcpu, u64 host_tsc,
+			      u64 target_tsc);
 void kvm_vcpu_write_tsc_offset(struct kvm_vcpu *vcpu, u64 l1_offset);
 
 static inline void adjust_tsc_offset_guest(struct kvm_vcpu *vcpu,
-- 
2.55.0.571.g244d577d93-goog