KVM: MMU: document fast page fault

Document fast page fault to Documentation/virtual/kvm/mmu.txt
Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virtual/kvm/mmu.txt

@@ -277,6 +277,9 @@ Handling a page fault is performed as follows:
    - walk shadow page table
    - cache the information to vcpu->arch.mmio_gva, vcpu->arch.access and
      vcpu->arch.mmio_gfn, and call the emulator
+ - If both P bit and R/W bit of error code are set, this could possibly
+   be handled as a "fast page fault" (fixed without taking the MMU lock).  See
+   the description in Documentation/virtual/kvm/locking.txt.
  - if needed, walk the guest page tables to determine the guest translation
    (gva->gpa or ngpa->gpa)
    - if permissions are insufficient, reflect the fault back to the guest