KVM: x86/mmu: Document dependency bewteen TDP A/D type and saved bits

Document that SHADOW_ACC_TRACK_SAVED_BITS_SHIFT is directly dependent on
bits 53:52 being used to track the A/D type.

Remove PT64_SECOND_AVAIL_BITS_SHIFT as it is at best misleading, and at
worst wrong.  For PAE paging, which arguably is a variant of PT64, the
bits are reserved.  For MMIO SPTEs the bits are not available as they're
used for the MMIO generation.  For access tracked SPTEs, they are also
not available as bits 56:54 are used to store the original RX bits.

No functional change intended.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210225204749.1512652-15-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/spte.h

@@ -6,7 +6,6 @@
 #include "mmu_internal.h"
 
 #define PT_FIRST_AVAIL_BITS_SHIFT 10
-#define PT64_SECOND_AVAIL_BITS_SHIFT 54
 
 /*
  * TDP SPTES (more specifically, EPT SPTEs) may not have A/D bits, and may also
@@ -134,11 +133,14 @@ extern u64 __read_mostly shadow_nonpresent_or_rsvd_mask;
  * The mask/shift to use for saving the original R/X bits when marking the PTE
  * as not-present for access tracking purposes. We do not save the W bit as the
  * PTEs being access tracked also need to be dirty tracked, so the W bit will be
- * restored only when a write is attempted to the page.
+ * restored only when a write is attempted to the page.  This mask obviously
+ * must not overlap the A/D type mask.
  */
 #define SHADOW_ACC_TRACK_SAVED_BITS_MASK (PT64_EPT_READABLE_MASK | \
 					  PT64_EPT_EXECUTABLE_MASK)
-#define SHADOW_ACC_TRACK_SAVED_BITS_SHIFT PT64_SECOND_AVAIL_BITS_SHIFT
+#define SHADOW_ACC_TRACK_SAVED_BITS_SHIFT 54
+static_assert(!(SPTE_TDP_AD_MASK & (SHADOW_ACC_TRACK_SAVED_BITS_MASK <<
+				    SHADOW_ACC_TRACK_SAVED_BITS_SHIFT)));
 
 /*
  * If a thread running without exclusive control of the MMU lock must perform a