kvm/x86 : add document for coalesced mmio

Signed-off-by: NPeng Hao <peng.hao2@zte.com.cn>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Documentation/virtual/kvm/api.txt

@@ -3681,6 +3681,30 @@ Returns: 0 on success, -1 on error
 This copies the vcpu's kvm_nested_state struct from userspace to the kernel.  For
 the definition of struct kvm_nested_state, see KVM_GET_NESTED_STATE.
 
+4.116 KVM_(UN)REGISTER_COALESCED_MMIO
+
+Capability: KVM_CAP_COALESCED_MMIO
+Architectures: all
+Type: vm ioctl
+Parameters: struct kvm_coalesced_mmio_zone
+Returns: 0 on success, < 0 on error
+
+Coalesced mmio is a performance optimization that defers hardware
+register write emulation so that userspace exits are avoided.  It is
+typically used to reduce the overhead of emulating frequently accessed
+hardware registers.
+
+When a hardware register is configured for coalesced mmio, write accesses
+do not exit to userspace and their value is recorded in a ring buffer
+that is shared between kernel and userspace.
+
+Coalesced mmio is used if one or more write accesses to a hardware
+register can be deferred until a read or a write to another hardware
+register on the same device.  This last access will cause a vmexit and
+userspace will process accesses from the ring buffer before emulating
+it. That will avoid exiting to userspace on repeated writes to the
+first register.
+
 5. The kvm_run structure
 ------------------------