kvm: x86: add tsc_offset field to struct kvm_vcpu_arch

A future commit will want to easily read a vCPU's TSC offset,
so we store it in struct kvm_arch_vcpu_arch for easy access.
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -568,6 +568,7 @@ struct kvm_vcpu_arch {
 		struct kvm_steal_time steal;
 	} st;
 
+	u64 tsc_offset;
 	u64 last_guest_tsc;
 	u64 last_host_tsc;
 	u64 tsc_offset_adjustment;

arch/x86/kvm/x86.c

@@ -1413,6 +1413,12 @@ u64 kvm_read_l1_tsc(struct kvm_vcpu *vcpu, u64 host_tsc)
 }
 EXPORT_SYMBOL_GPL(kvm_read_l1_tsc);
 
+static void kvm_vcpu_write_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
+{
+	kvm_x86_ops->write_tsc_offset(vcpu, offset);
+	vcpu->arch.tsc_offset = offset;
+}
+
 void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr)
 {
 	struct kvm *kvm = vcpu->kvm;
@@ -1522,7 +1528,7 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr)
 
 	if (guest_cpuid_has_tsc_adjust(vcpu) && !msr->host_initiated)
 		update_ia32_tsc_adjust_msr(vcpu, offset);
-	kvm_x86_ops->write_tsc_offset(vcpu, offset);
+	kvm_vcpu_write_tsc_offset(vcpu, offset);
 	raw_spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags);
 
 	spin_lock(&kvm->arch.pvclock_gtod_sync_lock);
@@ -2750,7 +2756,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 		if (check_tsc_unstable()) {
 			u64 offset = kvm_compute_tsc_offset(vcpu,
 						vcpu->arch.last_guest_tsc);
-			kvm_x86_ops->write_tsc_offset(vcpu, offset);
+			kvm_vcpu_write_tsc_offset(vcpu, offset);
 			vcpu->arch.tsc_catchup = 1;
 		}
 		/*