KVM: x86: Move TSC offset writes to common code

Also, ensure that the storing of the offset and the reading of the TSC
are never preempted by taking a spinlock.  While the lock is overkill
now, it is useful later in this patch series.

Signed-off-by: Zachary Amsden <zamsden@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -395,6 +395,7 @@ struct kvm_arch {
 
 	unsigned long irq_sources_bitmap;
 	s64 kvmclock_offset;
+	spinlock_t tsc_write_lock;
 
 	struct kvm_xen_hvm_config xen_hvm_config;
 
@@ -521,6 +522,8 @@ struct kvm_x86_ops {
 
 	bool (*has_wbinvd_exit)(void);
 
+	void (*write_tsc_offset)(struct kvm_vcpu *vcpu, u64 offset);
+
 	const struct trace_print_flags *exit_reasons_str;
 };