Documentation/kvm : Add documentation on Hypercalls and features used for PV spinlock

KVM_HC_KICK_CPU  hypercall added to wakeup halted vcpu in paravirtual spinlock
enabled guest.

KVM_FEATURE_PV_UNHALT enables guest to check whether pv spinlock can be enabled
in guest.

Thanks Vatsa for rewriting KVM_HC_KICK_CPU
Cc: Rob Landley <rob@landley.net>
Signed-off-by: NSrivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Signed-off-by: NRaghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Acked-by: NGleb Natapov <gleb@redhat.com>
Acked-by: NIngo Molnar <mingo@kernel.org>
Signed-off-by: NGleb Natapov <gleb@redhat.com>

Documentation/virtual/kvm/cpuid.txt

@@ -43,6 +43,10 @@ KVM_FEATURE_CLOCKSOURCE2           ||     3 || kvmclock available at msrs
 KVM_FEATURE_ASYNC_PF               ||     4 || async pf can be enabled by
                                    ||       || writing to msr 0x4b564d02
 ------------------------------------------------------------------------------
+KVM_FEATURE_PV_UNHALT              ||     7 || guest checks this feature bit
+                                   ||       || before enabling paravirtualized
+                                   ||       || spinlock support.
+------------------------------------------------------------------------------
 KVM_FEATURE_CLOCKSOURCE_STABLE_BIT ||    24 || host will warn if no guest-side
                                    ||       || per-cpu warps are expected in
                                    ||       || kvmclock.

Documentation/virtual/kvm/hypercalls.txt

@@ -64,3 +64,17 @@ Purpose: To enable communication between the hypervisor and guest there is a
 shared page that contains parts of supervisor visible register state.
 The guest can map this shared page to access its supervisor register through
 memory using this hypercall.
+
+5. KVM_HC_KICK_CPU
+------------------------
+Architecture: x86
+Status: active
+Purpose: Hypercall used to wakeup a vcpu from HLT state
+Usage example : A vcpu of a paravirtualized guest that is busywaiting in guest
+kernel mode for an event to occur (ex: a spinlock to become available) can
+execute HLT instruction once it has busy-waited for more than a threshold
+time-interval. Execution of HLT instruction would cause the hypervisor to put
+the vcpu to sleep until occurence of an appropriate event. Another vcpu of the
+same guest can wakeup the sleeping vcpu by issuing KVM_HC_KICK_CPU hypercall,
+specifying APIC ID (a1) of the vcpu to be woken up. An additional argument (a0)
+is used in the hypercall for future use.