KVM: Start lock documentation

The goal of this document shall be
- overview of all locks used in KVM core
- provide details on the scope of each lock
- explain the lock type, specifically of a raw spin locks
- provide a lock ordering guide
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NAvi Kivity <avi@redhat.com>

Documentation/kvm/locking.txt

+KVM Lock Overview
+=================
+
+1. Acquisition Orders
+---------------------
+
+(to be written)
+
+2. Reference
+------------
+
+Name:		kvm_lock
+Type:		raw_spinlock
+Arch:		any
+Protects:	- vm_list
+		- hardware virtualization enable/disable
+Comment:	'raw' because hardware enabling/disabling must be atomic /wrt
+		migration.
+
+Name:		kvm_arch::tsc_write_lock
+Type:		raw_spinlock
+Arch:		x86
+Protects:	- kvm_arch::{last_tsc_write,last_tsc_nsec,last_tsc_offset}
+		- tsc offset in vmcb
+Comment:	'raw' because updating the tsc offsets must not be preempted.