KVM: nVMX: Fix nested VMX TSC emulation

This patch fixes two corner cases in nested (L2) handling of TSC-related
issues:

1. Somewhat suprisingly, according to the Intel spec, if L1 allows WRMSR to
the TSC MSR without an exit, then this should set L1's TSC value itself - not
offset by vmcs12.TSC_OFFSET (like was wrongly done in the previous code).

2. Allow L1 to disable the TSC_OFFSETING control, and then correctly ignore
the vmcs12.TSC_OFFSET.
Signed-off-by: Nadav Har'El <nyh@il.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/kvm/vmx.c

@@ -1777,15 +1777,23 @@ static void vmx_set_tsc_khz(struct kvm_vcpu *vcpu, u32 user_tsc_khz)
  */
 static void vmx_write_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
 {
-	vmcs_write64(TSC_OFFSET, offset);
-	if (is_guest_mode(vcpu))
+	if (is_guest_mode(vcpu)) {
 		/*
-		 * We're here if L1 chose not to trap the TSC MSR. Since
-		 * prepare_vmcs12() does not copy tsc_offset, we need to also
-		 * set the vmcs12 field here.
+		 * 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.
 		 */
-		get_vmcs12(vcpu)->tsc_offset = offset -
-			to_vmx(vcpu)->nested.vmcs01_tsc_offset;
+		struct vmcs12 *vmcs12;
+		to_vmx(vcpu)->nested.vmcs01_tsc_offset = offset;
+		/* recalculate vmcs02.TSC_OFFSET: */
+		vmcs12 = get_vmcs12(vcpu);
+		vmcs_write64(TSC_OFFSET, offset +
+			(nested_cpu_has(vmcs12, CPU_BASED_USE_TSC_OFFSETING) ?
+			 vmcs12->tsc_offset : 0));
+	} else {
+		vmcs_write64(TSC_OFFSET, offset);
+	}
 }
 
 static void vmx_adjust_tsc_offset(struct kvm_vcpu *vcpu, s64 adjustment)
@@ -6485,8 +6493,11 @@ static void prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12)
 
 	set_cr4_guest_host_mask(vmx);
 
+	if (vmcs12->cpu_based_vm_exec_control & CPU_BASED_USE_TSC_OFFSETING)
 		vmcs_write64(TSC_OFFSET,
 			vmx->nested.vmcs01_tsc_offset + vmcs12->tsc_offset);
+	else
+		vmcs_write64(TSC_OFFSET, vmx->nested.vmcs01_tsc_offset);
 
 	if (enable_vpid) {
 		/*
@@ -6893,7 +6904,7 @@ static void nested_vmx_vmexit(struct kvm_vcpu *vcpu)
 
 	load_vmcs12_host_state(vcpu, vmcs12);
 
-	/* Update TSC_OFFSET if vmx_adjust_tsc_offset() was used while L2 ran */
+	/* Update TSC_OFFSET if TSC was changed while L2 ran */
 	vmcs_write64(TSC_OFFSET, vmx->nested.vmcs01_tsc_offset);
 
 	/* This is needed for same reason as it was needed in prepare_vmcs02 */