kvm: x86: mmu: allow A/D bits to be disabled in an mmu

Adds the plumbing to disable A/D bits in the MMU based on a new role
bit, ad_disabled. When A/D is disabled, the MMU operates as though A/D
aren't available (i.e., using access tracking faults instead).

To avoid SP -> kvm_mmu_page.role.ad_disabled lookups all over the
place, A/D disablement is now stored in the SPTE. This state is stored
in the SPTE by tweaking the use of SPTE_SPECIAL_MASK for access
tracking. Rather than just setting SPTE_SPECIAL_MASK when an
access-tracking SPTE is non-present, we now always set
SPTE_SPECIAL_MASK for access-tracking SPTEs.
Signed-off-by: Peter Feiner <pfeiner@google.com>
[Use role.ad_disabled even for direct (non-shadow) EPT page tables.  Add
 documentation and a few MMU_WARN_ONs. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virtual/kvm/mmu.txt

@@ -179,6 +179,10 @@ Shadow pages contain the following information:
     shadow page; it is also used to go back from a struct kvm_mmu_page
     to a memslot, through the kvm_memslots_for_spte_role macro and
     __gfn_to_memslot.
+  role.ad_disabled:
+    Is 1 if the MMU instance cannot use A/D bits.  EPT did not have A/D
+    bits before Haswell; shadow EPT page tables also cannot use A/D bits
+    if the L1 hypervisor does not enable them.
   gfn:
     Either the guest page table containing the translations shadowed by this
     page, or the base page frame for linear translations.  See role.direct.

arch/x86/include/asm/kvm_host.h

@@ -257,7 +257,8 @@ union kvm_mmu_page_role {
 		unsigned cr0_wp:1;
 		unsigned smep_andnot_wp:1;
 		unsigned smap_andnot_wp:1;
-		unsigned :8;
+		unsigned ad_disabled:1;
+		unsigned :7;
 
 		/*
 		 * This is left at the top of the word so that

arch/x86/kvm/mmu.c

@@ -187,10 +187,9 @@ static u64 __read_mostly shadow_mmio_value;
 static u64 __read_mostly shadow_present_mask;
 
 /*
- * The mask/value to distinguish a PTE that has been marked not-present for
- * access tracking purposes.
- * The mask would be either 0 if access tracking is disabled, or
- * SPTE_SPECIAL_MASK|VMX_EPT_RWX_MASK if access tracking is enabled.
+ * SPTEs used by MMUs without A/D bits are marked with shadow_acc_track_value.
+ * Non-present SPTEs with shadow_acc_track_value set are in place for access
+ * tracking.
  */
 static u64 __read_mostly shadow_acc_track_mask;
 static const u64 shadow_acc_track_value = SPTE_SPECIAL_MASK;
@@ -216,10 +215,32 @@ void kvm_mmu_set_mmio_spte_mask(u64 mmio_mask, u64 mmio_value)
 }
 EXPORT_SYMBOL_GPL(kvm_mmu_set_mmio_spte_mask);
 
+static inline bool sp_ad_disabled(struct kvm_mmu_page *sp)
+{
+	return sp->role.ad_disabled;
+}
+
+static inline bool spte_ad_enabled(u64 spte)
+{
+	MMU_WARN_ON((spte & shadow_mmio_mask) == shadow_mmio_value);
+	return !(spte & shadow_acc_track_value);
+}
+
+static inline u64 spte_shadow_accessed_mask(u64 spte)
+{
+	MMU_WARN_ON((spte & shadow_mmio_mask) == shadow_mmio_value);
+	return spte_ad_enabled(spte) ? shadow_accessed_mask : 0;
+}
+
+static inline u64 spte_shadow_dirty_mask(u64 spte)
+{
+	MMU_WARN_ON((spte & shadow_mmio_mask) == shadow_mmio_value);
+	return spte_ad_enabled(spte) ? shadow_dirty_mask : 0;
+}
+
 static inline bool is_access_track_spte(u64 spte)
 {
-	/* Always false if shadow_acc_track_mask is zero.  */
-	return (spte & shadow_acc_track_mask) == shadow_acc_track_value;
+	return !spte_ad_enabled(spte) && (spte & shadow_acc_track_mask) == 0;
 }
 
 /*
@@ -329,10 +350,9 @@ void kvm_mmu_set_mask_ptes(u64 user_mask, u64 accessed_mask,
 		u64 dirty_mask, u64 nx_mask, u64 x_mask, u64 p_mask,
 		u64 acc_track_mask)
 {
-	if (acc_track_mask != 0)
-		acc_track_mask |= SPTE_SPECIAL_MASK;
 	BUG_ON(!dirty_mask != !accessed_mask);
 	BUG_ON(!accessed_mask && !acc_track_mask);
+	BUG_ON(acc_track_mask & shadow_acc_track_value);
 
 	shadow_user_mask = user_mask;
 	shadow_accessed_mask = accessed_mask;
@@ -341,7 +361,6 @@ void kvm_mmu_set_mask_ptes(u64 user_mask, u64 accessed_mask,
 	shadow_x_mask = x_mask;
 	shadow_present_mask = p_mask;
 	shadow_acc_track_mask = acc_track_mask;
-	WARN_ON(shadow_accessed_mask != 0 && shadow_acc_track_mask != 0);
 }
 EXPORT_SYMBOL_GPL(kvm_mmu_set_mask_ptes);
 
@@ -561,7 +580,7 @@ static bool spte_has_volatile_bits(u64 spte)
 	    is_access_track_spte(spte))
 		return true;
 
-	if (shadow_accessed_mask) {
+	if (spte_ad_enabled(spte)) {
 		if ((spte & shadow_accessed_mask) == 0 ||
 	    	    (is_writable_pte(spte) && (spte & shadow_dirty_mask) == 0))
 			return true;
@@ -572,14 +591,17 @@ static bool spte_has_volatile_bits(u64 spte)
 
 static bool is_accessed_spte(u64 spte)
 {
-	return shadow_accessed_mask ? spte & shadow_accessed_mask
+	u64 accessed_mask = spte_shadow_accessed_mask(spte);
+
+	return accessed_mask ? spte & accessed_mask
 			     : !is_access_track_spte(spte);
 }
 
 static bool is_dirty_spte(u64 spte)
 {
-	return shadow_dirty_mask ? spte & shadow_dirty_mask
-				 : spte & PT_WRITABLE_MASK;
+	u64 dirty_mask = spte_shadow_dirty_mask(spte);
+
+	return dirty_mask ? spte & dirty_mask : spte & PT_WRITABLE_MASK;
 }
 
 /* Rules for using mmu_spte_set:
@@ -719,10 +741,10 @@ static u64 mmu_spte_get_lockless(u64 *sptep)
 
 static u64 mark_spte_for_access_track(u64 spte)
 {
-	if (shadow_accessed_mask != 0)
+	if (spte_ad_enabled(spte))
 		return spte & ~shadow_accessed_mask;
 
-	if (shadow_acc_track_mask == 0 || is_access_track_spte(spte))
+	if (is_access_track_spte(spte))
 		return spte;
 
 	/*
@@ -741,7 +763,6 @@ static u64 mark_spte_for_access_track(u64 spte)
 	spte |= (spte & shadow_acc_track_saved_bits_mask) <<
 		shadow_acc_track_saved_bits_shift;
 	spte &= ~shadow_acc_track_mask;
-	spte |= shadow_acc_track_value;
 
 	return spte;
 }
@@ -753,6 +774,7 @@ static u64 restore_acc_track_spte(u64 spte)
 	u64 saved_bits = (spte >> shadow_acc_track_saved_bits_shift)
 			 & shadow_acc_track_saved_bits_mask;
 
+	WARN_ON_ONCE(spte_ad_enabled(spte));
 	WARN_ON_ONCE(!is_access_track_spte(spte));
 
 	new_spte &= ~shadow_acc_track_mask;
@@ -771,7 +793,7 @@ static bool mmu_spte_age(u64 *sptep)
 	if (!is_accessed_spte(spte))
 		return false;
 
-	if (shadow_accessed_mask) {
+	if (spte_ad_enabled(spte)) {
 		clear_bit((ffs(shadow_accessed_mask) - 1),
 			  (unsigned long *)sptep);
 	} else {
@@ -1402,6 +1424,22 @@ static bool spte_clear_dirty(u64 *sptep)
 	return mmu_spte_update(sptep, spte);
 }
 
+static bool wrprot_ad_disabled_spte(u64 *sptep)
+{
+	bool was_writable = test_and_clear_bit(PT_WRITABLE_SHIFT,
+					       (unsigned long *)sptep);
+	if (was_writable)
+		kvm_set_pfn_dirty(spte_to_pfn(*sptep));
+
+	return was_writable;
+}
+
+/*
+ * Gets the GFN ready for another round of dirty logging by clearing the
+ *	- D bit on ad-enabled SPTEs, and
+ *	- W bit on ad-disabled SPTEs.
+ * Returns true iff any D or W bits were cleared.
+ */
 static bool __rmap_clear_dirty(struct kvm *kvm, struct kvm_rmap_head *rmap_head)
 {
 	u64 *sptep;
@@ -1409,7 +1447,10 @@ static bool __rmap_clear_dirty(struct kvm *kvm, struct kvm_rmap_head *rmap_head)
 	bool flush = false;
 
 	for_each_rmap_spte(rmap_head, &iter, sptep)
+		if (spte_ad_enabled(*sptep))
 			flush |= spte_clear_dirty(sptep);
+		else
+			flush |= wrprot_ad_disabled_spte(sptep);
 
 	return flush;
 }
@@ -1432,6 +1473,7 @@ static bool __rmap_set_dirty(struct kvm *kvm, struct kvm_rmap_head *rmap_head)
 	bool flush = false;
 
 	for_each_rmap_spte(rmap_head, &iter, sptep)
+		if (spte_ad_enabled(*sptep))
 			flush |= spte_set_dirty(sptep);
 
 	return flush;
@@ -1464,7 +1506,8 @@ static void kvm_mmu_write_protect_pt_masked(struct kvm *kvm,
 }
 
 /**
- * kvm_mmu_clear_dirty_pt_masked - clear MMU D-bit for PT level pages
+ * kvm_mmu_clear_dirty_pt_masked - clear MMU D-bit for PT level pages, or write
+ * protect the page if the D-bit isn't supported.
  * @kvm: kvm instance
  * @slot: slot to clear D-bit
  * @gfn_offset: start of the BITS_PER_LONG pages we care about
@@ -2389,7 +2432,12 @@ static void link_shadow_page(struct kvm_vcpu *vcpu, u64 *sptep,
 	BUILD_BUG_ON(VMX_EPT_WRITABLE_MASK != PT_WRITABLE_MASK);
 
 	spte = __pa(sp->spt) | shadow_present_mask | PT_WRITABLE_MASK |
-	       shadow_user_mask | shadow_x_mask | shadow_accessed_mask;
+	       shadow_user_mask | shadow_x_mask;
+
+	if (sp_ad_disabled(sp))
+		spte |= shadow_acc_track_value;
+	else
+		spte |= shadow_accessed_mask;
 
 	mmu_spte_set(sptep, spte);
 
@@ -2657,10 +2705,15 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
 {
 	u64 spte = 0;
 	int ret = 0;
+	struct kvm_mmu_page *sp;
 
 	if (set_mmio_spte(vcpu, sptep, gfn, pfn, pte_access))
 		return 0;
 
+	sp = page_header(__pa(sptep));
+	if (sp_ad_disabled(sp))
+		spte |= shadow_acc_track_value;
+
 	/*
 	 * For the EPT case, shadow_present_mask is 0 if hardware
 	 * supports exec-only page table entries.  In that case,
@@ -2669,7 +2722,7 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
 	 */
 	spte |= shadow_present_mask;
 	if (!speculative)
-		spte |= shadow_accessed_mask;
+		spte |= spte_shadow_accessed_mask(spte);
 
 	if (pte_access & ACC_EXEC_MASK)
 		spte |= shadow_x_mask;
@@ -2726,7 +2779,7 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
 
 	if (pte_access & ACC_WRITE_MASK) {
 		kvm_vcpu_mark_page_dirty(vcpu, gfn);
-		spte |= shadow_dirty_mask;
+		spte |= spte_shadow_dirty_mask(spte);
 	}
 
 	if (speculative)
@@ -2868,16 +2921,16 @@ static void direct_pte_prefetch(struct kvm_vcpu *vcpu, u64 *sptep)
 {
 	struct kvm_mmu_page *sp;
 
+	sp = page_header(__pa(sptep));
+
 	/*
-	 * Since it's no accessed bit on EPT, it's no way to
-	 * distinguish between actually accessed translations
-	 * and prefetched, so disable pte prefetch if EPT is
-	 * enabled.
+	 * Without accessed bits, there's no way to distinguish between
+	 * actually accessed translations and prefetched, so disable pte
+	 * prefetch if accessed bits aren't available.
 	 */
-	if (!shadow_accessed_mask)
+	if (sp_ad_disabled(sp))
 		return;
 
-	sp = page_header(__pa(sptep));
 	if (sp->role.level > PT_PAGE_TABLE_LEVEL)
 		return;
 
@@ -4278,6 +4331,7 @@ static void init_kvm_tdp_mmu(struct kvm_vcpu *vcpu)
 
 	context->base_role.word = 0;
 	context->base_role.smm = is_smm(vcpu);
+	context->base_role.ad_disabled = (shadow_accessed_mask == 0);
 	context->page_fault = tdp_page_fault;
 	context->sync_page = nonpaging_sync_page;
 	context->invlpg = nonpaging_invlpg;
@@ -4624,6 +4678,7 @@ static void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
 	mask.smep_andnot_wp = 1;
 	mask.smap_andnot_wp = 1;
 	mask.smm = 1;
+	mask.ad_disabled = 1;
 
 	/*
 	 * If we don't have indirect shadow pages, it means no page is

arch/x86/kvm/mmutrace.h

@@ -30,8 +30,9 @@
 								        \
 	role.word = __entry->role;					\
 									\
-	trace_seq_printf(p, "sp gen %lx gfn %llx %u%s q%u%s %s%s"	\
-			 " %snxe root %u %s%c",	__entry->mmu_valid_gen,	\
+	trace_seq_printf(p, "sp gen %lx gfn %llx l%u%s q%u%s %s%s"	\
+			 " %snxe %sad root %u %s%c",			\
+			 __entry->mmu_valid_gen,			\
 			 __entry->gfn, role.level,			\
 			 role.cr4_pae ? " pae" : "",			\
 			 role.quadrant,					\
@@ -39,6 +40,7 @@
 			 access_str[role.access],			\
 			 role.invalid ? " invalid" : "",		\
 			 role.nxe ? "" : "!",				\
+			 role.ad_disabled ? "!" : "",			\
 			 __entry->root_count,				\
 			 __entry->unsync ? "unsync" : "sync", 0);	\
 	saved_ptr;							\