KVM: x86: fix handling of role.cr4_pae and rename it to 'gpte_size'

The cr4_pae flag is a bit of a misnomer, its purpose is really to track
whether the guest PTE that is being shadowed is a 4-byte entry or an
8-byte entry.  Prior to supporting nested EPT, the size of the gpte was
reflected purely by CR4.PAE.  KVM fudged things a bit for direct sptes,
but it was mostly harmless since the size of the gpte never mattered.
Now that a spte may be tracking an indirect EPT entry, relying on
CR4.PAE is wrong and ill-named.

For direct shadow pages, force the gpte_size to '1' as they are always
8-byte entries; EPT entries can only be 8-bytes and KVM always uses
8-byte entries for NPT and its identity map (when running with EPT but
not unrestricted guest).

Likewise, nested EPT entries are always 8-bytes.  Nested EPT presents a
unique scenario as the size of the entries are not dictated by CR4.PAE,
but neither is the shadow page a direct map.  To handle this scenario,
set cr0_wp=1 and smap_andnot_wp=1, an otherwise impossible combination,
to denote a nested EPT shadow page.  Use the information to avoid
incorrectly zapping an unsync'd indirect page in __kvm_sync_page().

Providing a consistent and accurate gpte_size fixes a bug reported by
Vitaly where fast_cr3_switch() always fails when switching from L2 to
L1 as kvm_mmu_get_page() would force role.cr4_pae=0 for direct pages,
whereas kvm_calc_mmu_role_common() would set it according to CR4.PAE.

Fixes: 7dcd575 ("x86/kvm/mmu: check if tdp/shadow MMU reconfiguration is needed")
Reported-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Tested-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virtual/kvm/mmu.txt

@@ -142,7 +142,7 @@ Shadow pages contain the following information:
     If clear, this page corresponds to a guest page table denoted by the gfn
     field.
   role.quadrant:
-    When role.cr4_pae=0, the guest uses 32-bit gptes while the host uses 64-bit
+    When role.gpte_is_8_bytes=0, the guest uses 32-bit gptes while the host uses 64-bit
     sptes.  That means a guest page table contains more ptes than the host,
     so multiple shadow pages are needed to shadow one guest page.
     For first-level shadow pages, role.quadrant can be 0 or 1 and denotes the
@@ -158,9 +158,9 @@ Shadow pages contain the following information:
     The page is invalid and should not be used.  It is a root page that is
     currently pinned (by a cpu hardware register pointing to it); once it is
     unpinned it will be destroyed.
-  role.cr4_pae:
-    Contains the value of cr4.pae for which the page is valid (e.g. whether
-    32-bit or 64-bit gptes are in use).
+  role.gpte_is_8_bytes:
+    Reflects the size of the guest PTE for which the page is valid, i.e. '1'
+    if 64-bit gptes are in use, '0' if 32-bit gptes are in use.
   role.nxe:
     Contains the value of efer.nxe for which the page is valid.
   role.cr0_wp:
@@ -173,6 +173,9 @@ Shadow pages contain the following information:
     Contains the value of cr4.smap && !cr0.wp for which the page is valid
     (pages for which this is true are different from other pages; see the
     treatment of cr0.wp=0 below).
+  role.ept_sp:
+    This is a virtual flag to denote a shadowed nested EPT page.  ept_sp
+    is true if "cr0_wp && smap_andnot_wp", an otherwise invalid combination.
   role.smm:
     Is 1 if the page is valid in system management mode.  This field
     determines which of the kvm_memslots array was used to build this

arch/x86/include/asm/kvm_host.h

@@ -253,14 +253,14 @@ struct kvm_mmu_memory_cache {
  * kvm_memory_slot.arch.gfn_track which is 16 bits, so the role bits used
  * by indirect shadow page can not be more than 15 bits.
  *
- * Currently, we used 14 bits that are @level, @cr4_pae, @quadrant, @access,
+ * Currently, we used 14 bits that are @level, @gpte_is_8_bytes, @quadrant, @access,
  * @nxe, @cr0_wp, @smep_andnot_wp and @smap_andnot_wp.
  */
 union kvm_mmu_page_role {
 	u32 word;
 	struct {
 		unsigned level:4;
-		unsigned cr4_pae:1;
+		unsigned gpte_is_8_bytes:1;
 		unsigned quadrant:2;
 		unsigned direct:1;
 		unsigned access:3;

arch/x86/kvm/mmu.c

@@ -182,7 +182,7 @@ struct kvm_shadow_walk_iterator {
 
 static const union kvm_mmu_page_role mmu_base_role_mask = {
 	.cr0_wp = 1,
-	.cr4_pae = 1,
+	.gpte_is_8_bytes = 1,
 	.nxe = 1,
 	.smep_andnot_wp = 1,
 	.smap_andnot_wp = 1,
@@ -2205,6 +2205,7 @@ static bool kvm_mmu_prepare_zap_page(struct kvm *kvm, struct kvm_mmu_page *sp,
 static void kvm_mmu_commit_zap_page(struct kvm *kvm,
 				    struct list_head *invalid_list);
 
+
 #define for_each_valid_sp(_kvm, _sp, _gfn)				\
 	hlist_for_each_entry(_sp,					\
 	  &(_kvm)->arch.mmu_page_hash[kvm_page_table_hashfn(_gfn)], hash_link) \
@@ -2215,12 +2216,17 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm,
 	for_each_valid_sp(_kvm, _sp, _gfn)				\
 		if ((_sp)->gfn != (_gfn) || (_sp)->role.direct) {} else
 
+static inline bool is_ept_sp(struct kvm_mmu_page *sp)
+{
+	return sp->role.cr0_wp && sp->role.smap_andnot_wp;
+}
+
 /* @sp->gfn should be write-protected at the call site */
 static bool __kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
 			    struct list_head *invalid_list)
 {
-	if (sp->role.cr4_pae != !!is_pae(vcpu)
-	    || vcpu->arch.mmu->sync_page(vcpu, sp) == 0) {
+	if ((!is_ept_sp(sp) && sp->role.gpte_is_8_bytes != !!is_pae(vcpu)) ||
+	    vcpu->arch.mmu->sync_page(vcpu, sp) == 0) {
 		kvm_mmu_prepare_zap_page(vcpu->kvm, sp, invalid_list);
 		return false;
 	}
@@ -2423,7 +2429,7 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
 	role.level = level;
 	role.direct = direct;
 	if (role.direct)
-		role.cr4_pae = 0;
+		role.gpte_is_8_bytes = true;
 	role.access = access;
 	if (!vcpu->arch.mmu->direct_map
 	    && vcpu->arch.mmu->root_level <= PT32_ROOT_LEVEL) {
@@ -4794,7 +4800,6 @@ static union kvm_mmu_role kvm_calc_mmu_role_common(struct kvm_vcpu *vcpu,
 
 	role.base.access = ACC_ALL;
 	role.base.nxe = !!is_nx(vcpu);
-	role.base.cr4_pae = !!is_pae(vcpu);
 	role.base.cr0_wp = is_write_protection(vcpu);
 	role.base.smm = is_smm(vcpu);
 	role.base.guest_mode = is_guest_mode(vcpu);
@@ -4815,6 +4820,7 @@ kvm_calc_tdp_mmu_root_page_role(struct kvm_vcpu *vcpu, bool base_only)
 	role.base.ad_disabled = (shadow_accessed_mask == 0);
 	role.base.level = kvm_x86_ops->get_tdp_level(vcpu);
 	role.base.direct = true;
+	role.base.gpte_is_8_bytes = true;
 
 	return role;
 }
@@ -4879,6 +4885,7 @@ kvm_calc_shadow_mmu_root_page_role(struct kvm_vcpu *vcpu, bool base_only)
 	role.base.smap_andnot_wp = role.ext.cr4_smap &&
 		!is_write_protection(vcpu);
 	role.base.direct = !is_paging(vcpu);
+	role.base.gpte_is_8_bytes = !!is_pae(vcpu);
 
 	if (!is_long_mode(vcpu))
 		role.base.level = PT32E_ROOT_LEVEL;
@@ -4919,21 +4926,24 @@ kvm_calc_shadow_ept_root_page_role(struct kvm_vcpu *vcpu, bool accessed_dirty,
 				   bool execonly)
 {
 	union kvm_mmu_role role = {0};
-	union kvm_mmu_page_role root_base = vcpu->arch.root_mmu.mmu_role.base;
 
-	/* Legacy paging and SMM flags are inherited from root_mmu */
-	role.base.smm = root_base.smm;
-	role.base.nxe = root_base.nxe;
-	role.base.cr0_wp = root_base.cr0_wp;
-	role.base.smep_andnot_wp = root_base.smep_andnot_wp;
-	role.base.smap_andnot_wp = root_base.smap_andnot_wp;
+	/* SMM flag is inherited from root_mmu */
+	role.base.smm = vcpu->arch.root_mmu.mmu_role.base.smm;
 
 	role.base.level = PT64_ROOT_4LEVEL;
+	role.base.gpte_is_8_bytes = true;
 	role.base.direct = false;
 	role.base.ad_disabled = !accessed_dirty;
 	role.base.guest_mode = true;
 	role.base.access = ACC_ALL;
 
+	/*
+	 * WP=1 and NOT_WP=1 is an impossible combination, use WP and the
+	 * SMAP variation to denote shadow EPT entries.
+	 */
+	role.base.cr0_wp = true;
+	role.base.smap_andnot_wp = true;
+
 	role.ext = kvm_calc_mmu_role_ext(vcpu);
 	role.ext.execonly = execonly;
 
@@ -5184,7 +5194,7 @@ static bool detect_write_misaligned(struct kvm_mmu_page *sp, gpa_t gpa,
 		 gpa, bytes, sp->role.word);
 
 	offset = offset_in_page(gpa);
-	pte_size = sp->role.cr4_pae ? 8 : 4;
+	pte_size = sp->role.gpte_is_8_bytes ? 8 : 4;
 
 	/*
 	 * Sometimes, the OS only writes the last one bytes to update status
@@ -5208,7 +5218,7 @@ static u64 *get_written_sptes(struct kvm_mmu_page *sp, gpa_t gpa, int *nspte)
 	page_offset = offset_in_page(gpa);
 	level = sp->role.level;
 	*nspte = 1;
-	if (!sp->role.cr4_pae) {
+	if (!sp->role.gpte_is_8_bytes) {
 		page_offset <<= 1;	/* 32->64 */
 		/*
 		 * A 32-bit pde maps 4MB while the shadow pdes map

arch/x86/kvm/mmutrace.h

@@ -29,10 +29,10 @@
 								        \
 	role.word = __entry->role;					\
 									\
-	trace_seq_printf(p, "sp gfn %llx l%u%s q%u%s %s%s"		\
+	trace_seq_printf(p, "sp gfn %llx l%u %u-byte q%u%s %s%s"	\
 			 " %snxe %sad root %u %s%c",			\
 			 __entry->gfn, role.level,			\
-			 role.cr4_pae ? " pae" : "",			\
+			 role.gpte_is_8_bytes ? 8 : 4,			\
 			 role.quadrant,					\
 			 role.direct ? " direct" : "",			\
 			 access_str[role.access],			\