linux-kernel - [PATCH] KVM: x86: Trace changes to active TSC offset regardless if vCPU in guest-mode

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Date:   Sun, 25 Nov 2018 18:53:25 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Liran Alon <liran.alon@...cle.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: [PATCH] KVM: x86: Trace changes to active TSC offset regardless if vCPU in guest-mode

For some reason, kvm_x86_ops->write_l1_tsc_offset() skipped trace
of change to active TSC offset in case vCPU is in guest-mode.
This patch changes write_l1_tsc_offset() behavior to trace any change
to active TSC offset to aid debugging.  The VMX code is changed to
look more similar to SVM, which is in my opinion nicer.

Based on a patch by Liran Alon.

Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
	Untested still, but throwing it out because it seems pretty
	obvious...

 arch/x86/kvm/svm.c |  9 +++++----
 arch/x86/kvm/vmx.c | 34 +++++++++++++++++-----------------
 2 files changed, 22 insertions(+), 21 deletions(-)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index a24733aade4c..0d1a74069a9e 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1456,10 +1456,11 @@ static u64 svm_write_l1_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
 		g_tsc_offset = svm->vmcb->control.tsc_offset -
 			       svm->nested.hsave->control.tsc_offset;
 		svm->nested.hsave->control.tsc_offset = offset;
-	} else
-		trace_kvm_write_tsc_offset(vcpu->vcpu_id,
-					   svm->vmcb->control.tsc_offset,
-					   offset);
+	}
+
+	trace_kvm_write_tsc_offset(vcpu->vcpu_id,
+				   svm->vmcb->control.tsc_offset - g_tsc_offset,
+				   offset);
 
 	svm->vmcb->control.tsc_offset = offset + g_tsc_offset;
 
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 764c23dc444f..e7d3f7d35355 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -3466,24 +3466,24 @@ static u64 vmx_read_l1_tsc_offset(struct kvm_vcpu *vcpu)
 
 static u64 vmx_write_l1_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
 {
-	u64 active_offset = offset;
-	if (is_guest_mode(vcpu)) {
-		/*
-		 * We're here if L1 chose not to trap WRMSR to TSC. According
-		 * to the spec, this should set L1's TSC; The offset that L1
-		 * set for L2 remains unchanged, and still needs to be added
-		 * to the newly set TSC to get L2's TSC.
-		 */
-		struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
-		if (nested_cpu_has(vmcs12, CPU_BASED_USE_TSC_OFFSETING))
-			active_offset += vmcs12->tsc_offset;
-	} else {
-		trace_kvm_write_tsc_offset(vcpu->vcpu_id,
-					   vmcs_read64(TSC_OFFSET), offset);
-	}
+	struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
+	u64 g_tsc_offset = 0;
+
+	/*
+	 * We're here if L1 chose not to trap WRMSR to TSC. According
+	 * to the spec, this should set L1's TSC; The offset that L1
+	 * set for L2 remains unchanged, and still needs to be added
+	 * to the newly set TSC to get L2's TSC.
+	 */
+	if (is_guest_mode(vcpu) &&
+	    (vmcs12->cpu_based_vm_exec_control & CPU_BASED_USE_TSC_OFFSETING))
+		g_tsc_offset = vmcs12->tsc_offset;
 
-	vmcs_write64(TSC_OFFSET, active_offset);
-	return active_offset;
+	trace_kvm_write_tsc_offset(vcpu->vcpu_id,
+				   vcpu->arch.tsc_offset - g_tsc_offset,
+				   offset);
+	vmcs_write64(TSC_OFFSET, offset + g_tsc_offset);
+	return offset + g_tsc_offset;
 }
 
 /*
-- 
1.8.3.1