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: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

diff --git a/Documentation/kvm/locking.txt b/Documentation/kvm/locking.txt
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+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.