KVM: x86/mmu: Optimize and clean up so called "last nonleaf level" logic

Drop the pre-computed last_nonleaf_level, which is arguably wrong and at
best confusing.  Per the comment:

  Can have large pages at levels 2..last_nonleaf_level-1.

the intent of the variable would appear to be to track what levels can
_legally_ have large pages, but that intent doesn't align with reality.
The computed value will be wrong for 5-level paging, or if 1gb pages are
not supported.

The flawed code is not a problem in practice, because except for 32-bit
PSE paging, bit 7 is reserved if large pages aren't supported at the
level.  Take advantage of this invariant and simply omit the level magic
math for 64-bit page tables (including PAE).

For 32-bit paging (non-PAE), the adjustments are needed purely because
bit 7 is ignored if PSE=0.  Retain that logic as is, but make
is_last_gpte() unique per PTTYPE so that the PSE check is avoided for
PAE and EPT paging.  In the spirit of avoiding branches, bump the "last
nonleaf level" for 32-bit PSE paging by adding the PSE bit itself.

Note, bit 7 is ignored or has other meaning in CR3/EPTP, but despite
FNAME(walk_addr_generic) briefly grabbing CR3/EPTP in "pte", they are
not PTEs and will blow up all the other gpte helpers.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622175739.3610207-51-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -457,9 +457,6 @@ struct kvm_mmu {
 
 	struct rsvd_bits_validate guest_rsvd_check;
 
-	/* Can have large pages at levels 2..last_nonleaf_level-1. */
-	u8 last_nonleaf_level;
-
 	u64 pdptrs[4]; /* pae */
 };
 

arch/x86/kvm/mmu/mmu.c

@@ -4071,26 +4071,6 @@ static bool sync_mmio_spte(struct kvm_vcpu *vcpu, u64 *sptep, gfn_t gfn,
 	return false;
 }
 
-static inline bool is_last_gpte(struct kvm_mmu *mmu,
-				unsigned level, unsigned gpte)
-{
-	/*
-	 * The RHS has bit 7 set iff level < mmu->last_nonleaf_level.
-	 * If it is clear, there are no large pages at this level, so clear
-	 * PT_PAGE_SIZE_MASK in gpte if that is the case.
-	 */
-	gpte &= level - mmu->last_nonleaf_level;
-
-	/*
-	 * PG_LEVEL_4K always terminates.  The RHS has bit 7 set
-	 * iff level <= PG_LEVEL_4K, which for our purpose means
-	 * level == PG_LEVEL_4K; set PT_PAGE_SIZE_MASK in gpte then.
-	 */
-	gpte |= level - PG_LEVEL_4K - 1;
-
-	return gpte & PT_PAGE_SIZE_MASK;
-}
-
 #define PTTYPE_EPT 18 /* arbitrary */
 #define PTTYPE PTTYPE_EPT
 #include "paging_tmpl.h"
@@ -4491,15 +4471,6 @@ static void update_pkru_bitmask(struct kvm_mmu *mmu)
 	}
 }
 
-static void update_last_nonleaf_level(struct kvm_mmu *mmu)
-{
-	unsigned root_level = mmu->root_level;
-
-	mmu->last_nonleaf_level = root_level;
-	if (root_level == PT32_ROOT_LEVEL && is_cr4_pse(mmu))
-		mmu->last_nonleaf_level++;
-}
-
 static void reset_guest_paging_metadata(struct kvm_vcpu *vcpu,
 					struct kvm_mmu *mmu)
 {
@@ -4509,7 +4480,6 @@ static void reset_guest_paging_metadata(struct kvm_vcpu *vcpu,
 	reset_rsvds_bits_mask(vcpu, mmu);
 	update_permission_bitmask(mmu, false);
 	update_pkru_bitmask(mmu);
-	update_last_nonleaf_level(mmu);
 }
 
 static void paging64_init_context(struct kvm_mmu *context)
@@ -4783,7 +4753,6 @@ void kvm_init_shadow_ept_mmu(struct kvm_vcpu *vcpu, bool execonly,
 	context->direct_map = false;
 
 	update_permission_bitmask(context, true);
-	update_last_nonleaf_level(context);
 	update_pkru_bitmask(context);
 	reset_rsvds_bits_mask_ept(vcpu, context, execonly);
 	reset_ept_shadow_zero_bits_mask(vcpu, context, execonly);

arch/x86/kvm/mmu/paging_tmpl.h

@@ -305,6 +305,35 @@ static inline unsigned FNAME(gpte_pkeys)(struct kvm_vcpu *vcpu, u64 gpte)
 	return pkeys;
 }
 
+static inline bool FNAME(is_last_gpte)(struct kvm_mmu *mmu,
+				       unsigned int level, unsigned int gpte)
+{
+	/*
+	 * For EPT and PAE paging (both variants), bit 7 is either reserved at
+	 * all level or indicates a huge page (ignoring CR3/EPTP).  In either
+	 * case, bit 7 being set terminates the walk.
+	 */
+#if PTTYPE == 32
+	/*
+	 * 32-bit paging requires special handling because bit 7 is ignored if
+	 * CR4.PSE=0, not reserved.  Clear bit 7 in the gpte if the level is
+	 * greater than the last level for which bit 7 is the PAGE_SIZE bit.
+	 *
+	 * The RHS has bit 7 set iff level < (2 + PSE).  If it is clear, bit 7
+	 * is not reserved and does not indicate a large page at this level,
+	 * so clear PT_PAGE_SIZE_MASK in gpte if that is the case.
+	 */
+	gpte &= level - (PT32_ROOT_LEVEL + mmu->mmu_role.ext.cr4_pse);
+#endif
+	/*
+	 * PG_LEVEL_4K always terminates.  The RHS has bit 7 set
+	 * iff level <= PG_LEVEL_4K, which for our purpose means
+	 * level == PG_LEVEL_4K; set PT_PAGE_SIZE_MASK in gpte then.
+	 */
+	gpte |= level - PG_LEVEL_4K - 1;
+
+	return gpte & PT_PAGE_SIZE_MASK;
+}
 /*
  * Fetch a guest pte for a guest virtual address, or for an L2's GPA.
  */
@@ -421,7 +450,7 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker,
 
 		/* Convert to ACC_*_MASK flags for struct guest_walker.  */
 		walker->pt_access[walker->level - 1] = FNAME(gpte_access)(pt_access ^ walk_nx_mask);
-	} while (!is_last_gpte(mmu, walker->level, pte));
+	} while (!FNAME(is_last_gpte)(mmu, walker->level, pte));
 
 	pte_pkey = FNAME(gpte_pkeys)(vcpu, pte);
 	accessed_dirty = have_ad ? pte_access & PT_GUEST_ACCESSED_MASK : 0;