kvm: x86: mmu: Update documentation for fast page fault mechanism

Add a brief description of the lockless access tracking mechanism
to the documentation of fast page faults in locking.txt.
Signed-off-by: Junaid Shahid <junaids@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virtual/kvm/locking.txt

@@ -26,9 +26,16 @@ sections.
 Fast page fault:
 
 Fast page fault is the fast path which fixes the guest page fault out of
-the mmu-lock on x86. Currently, the page fault can be fast only if the
-shadow page table is present and it is caused by write-protect, that means
-we just need change the W bit of the spte.
+the mmu-lock on x86. Currently, the page fault can be fast in one of the
+following two cases:
+
+1. Access Tracking: The SPTE is not present, but it is marked for access
+tracking i.e. the SPTE_SPECIAL_MASK is set. That means we need to
+restore the saved R/X bits. This is described in more detail later below.
+
+2. Write-Protection: The SPTE is present and the fault is
+caused by write-protect. That means we just need to change the W bit of the 
+spte.
 
 What we use to avoid all the race is the SPTE_HOST_WRITEABLE bit and
 SPTE_MMU_WRITEABLE bit on the spte:
@@ -38,7 +45,8 @@ SPTE_MMU_WRITEABLE bit on the spte:
   page write-protection.
 
 On fast page fault path, we will use cmpxchg to atomically set the spte W
-bit if spte.SPTE_HOST_WRITEABLE = 1 and spte.SPTE_WRITE_PROTECT = 1, this
+bit if spte.SPTE_HOST_WRITEABLE = 1 and spte.SPTE_WRITE_PROTECT = 1, or 
+restore the saved R/X bits if VMX_EPT_TRACK_ACCESS mask is set, or both. This
 is safe because whenever changing these bits can be detected by cmpxchg.
 
 But we need carefully check these cases:
@@ -142,6 +150,21 @@ Since the spte is "volatile" if it can be updated out of mmu-lock, we always
 atomically update the spte, the race caused by fast page fault can be avoided,
 See the comments in spte_has_volatile_bits() and mmu_spte_update().
 
+Lockless Access Tracking:
+
+This is used for Intel CPUs that are using EPT but do not support the EPT A/D
+bits. In this case, when the KVM MMU notifier is called to track accesses to a
+page (via kvm_mmu_notifier_clear_flush_young), it marks the PTE as not-present
+by clearing the RWX bits in the PTE and storing the original R & X bits in
+some unused/ignored bits. In addition, the SPTE_SPECIAL_MASK is also set on the
+PTE (using the ignored bit 62). When the VM tries to access the page later on,
+a fault is generated and the fast page fault mechanism described above is used
+to atomically restore the PTE to a Present state. The W bit is not saved when
+the PTE is marked for access tracking and during restoration to the Present
+state, the W bit is set depending on whether or not it was a write access. If
+it wasn't, then the W bit will remain clear until a write access happens, at 
+which time it will be set using the Dirty tracking mechanism described above.
+
 3. Reference
 ------------