[PATCH 1/3] KVM: x86/mmu: Avoid subtle pointer arithmetic in kvm_mmu_child_role()

[Sean Christopherson <seanjc@google.com>]

When computing the quadrant (really the semicircle) for pages that shadow
4-byte guest Page Tables, grab the least significant bit of the PDE index
by using @sptep as if it were an index into an array, which it more or
less is.  Computing the PDE index using pointer arithmetic is subtle as
it relies on the pointer being a "u64 *", and is more expensive as the
compiler must perform the subtraction since the compiler doesn't know
that sptep and parent_sp->spt are tightly coupled.  Using only the value
of sptep allows the compiler to encode the computation as a SHR+AND.

Opportunstically update the comment to explicitly call out how and why
KVM uses role.quadrant to consume gPTE bits, and wrap an unnecessarily
long line.

No functional change intended.

Link: https://lore.kernel.org/all/YqvWvBv27fYzOFdE@google.com
Cc: David Matlack <dmatlack@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/mmu/mmu.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index bd74a287b54a..07dfed427d5b 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/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;
@@ -2195,13 +2196,19 @@ 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);
-		role.quadrant = (sptep - parent_sp->spt) % 2;
+		role.quadrant = ((unsigned long)sptep / sizeof(*sptep)) & 1;
 	}
 
 	return role;
-- 
2.37.0.rc0.161.g10f37bed90-goog