KVM: x86/mmu: Expand quadrant comment for PG_LEVEL_4K shadow pages

Tweak the comment above the computation of the quadrant for PG_LEVEL_4K
shadow pages to explicitly call out how and why KVM uses role.quadrant to
consume gPTE bits.

Opportunistically wrap an unnecessarily long line.

No functional change intended.

Link: https://lore.kernel.org/all/YqvWvBv27fYzOFdE@google.comReviewed-by: David Matlack <dmatlack@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220712020724.1262121-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -2168,7 +2168,8 @@ static struct kvm_mmu_page *kvm_mmu_get_shadow_page(struct kvm_vcpu *vcpu,
 	return __kvm_mmu_get_shadow_page(vcpu->kvm, vcpu, &caches, gfn, role);
 }
 
-static union kvm_mmu_page_role kvm_mmu_child_role(u64 *sptep, bool direct, unsigned int access)
+static union kvm_mmu_page_role kvm_mmu_child_role(u64 *sptep, bool direct,
+						  unsigned int access)
 {
 	struct kvm_mmu_page *parent_sp = sptep_to_sp(sptep);
 	union kvm_mmu_page_role role;
@@ -2195,9 +2196,15 @@ static union kvm_mmu_page_role kvm_mmu_child_role(u64 *sptep, bool direct, unsig
 	 * uses 2 PAE page tables, each mapping a 2MiB region. For these,
 	 * @role.quadrant encodes which half of the region they map.
 	 *
-	 * Note, the 4 PAE page directories are pre-allocated and the quadrant
-	 * assigned in mmu_alloc_root(). So only page tables need to be handled
-	 * here.
+	 * Concretely, a 4-byte PDE consumes bits 31:22, while an 8-byte PDE
+	 * consumes bits 29:21.  To consume bits 31:30, KVM's uses 4 shadow
+	 * PDPTEs; those 4 PAE page directories are pre-allocated and their
+	 * quadrant is assigned in mmu_alloc_root().   A 4-byte PTE consumes
+	 * bits 21:12, while an 8-byte PTE consumes bits 20:12.  To consume
+	 * bit 21 in the PTE (the child here), KVM propagates that bit to the
+	 * quadrant, i.e. sets quadrant to '0' or '1'.  The parent 8-byte PDE
+	 * covers bit 21 (see above), thus the quadrant is calculated from the
+	 * _least_ significant bit of the PDE index.
 	 */
 	if (role.has_4_byte_gpte) {
 		WARN_ON_ONCE(role.level != PG_LEVEL_4K);