Linux KVM Hypercall:
===================
X86:
 KVM Hypercalls have a three-byte sequence of either the vmcall or the vmmcall
 instruction. The hypervisor can replace it with instructions that are
 guaranteed to be supported.

 Up to four arguments may be passed in rbx, rcx, rdx, and rsi respectively.
 The hypercall number should be placed in rax and the return value will be
 placed in rax.  No other registers will be clobbered unless explicitly stated
 by the particular hypercall.

S390:
  R2-R7 are used for parameters 1-6. In addition, R1 is used for hypercall
  number. The return value is written to R2.

  S390 uses diagnose instruction as hypercall (0x500) along with hypercall
  number in R1.

  For further information on the S390 diagnose call as supported by KVM,
  refer to Documentation/virtual/kvm/s390-diag.txt.

 PowerPC:
  It uses R3-R10 and hypercall number in R11. R4-R11 are used as output registers.
  Return value is placed in R3.

  KVM hypercalls uses 4 byte opcode, that are patched with 'hypercall-instructions'
  property inside the device tree's /hypervisor node.
  For more information refer to Documentation/virtual/kvm/ppc-pv.txt

KVM Hypercalls Documentation
===========================
The template for each hypercall is:
1. Hypercall name.
2. Architecture(s)
3. Status (deprecated, obsolete, active)
4. Purpose

1. KVM_HC_VAPIC_POLL_IRQ
------------------------
Architecture: x86
Status: active
Purpose: Trigger guest exit so that the host can check for pending
interrupts on reentry.

2. KVM_HC_MMU_OP
------------------------
Architecture: x86
Status: deprecated.
Purpose: Support MMU operations such as writing to PTE,
flushing TLB, release PT.

3. KVM_HC_FEATURES
------------------------
Architecture: PPC
Status: active
Purpose: Expose hypercall availability to the guest. On x86 platforms, cpuid
used to enumerate which hypercalls are available. On PPC, either device tree
based lookup ( which is also what EPAPR dictates) OR KVM specific enumeration
mechanism (which is this hypercall) can be used.

4. KVM_HC_PPC_MAP_MAGIC_PAGE
------------------------
Architecture: PPC
Status: active
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 occurrence 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.