Re: [PATCH v10 7/8] KVM: x86/mmu: Track if sptes refer to refcounted pages

Paolo Bonzini <pbonzini@xxxxxxxxxx> · Fri, 23 Feb 2024 11:00:37 +0100

On 2/21/24 08:25, David Stevens wrote:
From: David Stevens <stevensd@xxxxxxxxxxxx>

Use one of the unused bits in EPT sptes to track whether or not an spte
refers to a struct page that has a valid refcount, in preparation for
adding support for mapping such pages into guests. The new bit is used
to avoid triggering a page_count() == 0 warning and to avoid touching
A/D bits of unknown usage.

Non-EPT sptes don't have any free bits to use, so this tracking is not
possible when TDP is disabled or on 32-bit x86.

TDX will add support for non-zero non-present PTEs.  We could use this 
to use inverted bit 8 to mark present PTEs (bit 8 set for non-present, 
bit 8 clear for present) for both shadow paging and AMD NPT.  This would 
free bit 11 for SPTE_MMU_PAGE_REFCOUNTED.

No need to do it now, though.

Paolo

Signed-off-by: David Stevens <stevensd@xxxxxxxxxxxx>
---
  arch/x86/kvm/mmu/mmu.c         | 43 +++++++++++++++++++---------------
  arch/x86/kvm/mmu/paging_tmpl.h |  5 ++--
  arch/x86/kvm/mmu/spte.c        |  4 +++-
  arch/x86/kvm/mmu/spte.h        | 22 ++++++++++++++++-
  arch/x86/kvm/mmu/tdp_mmu.c     | 21 ++++++++++-------
  include/linux/kvm_host.h       |  3 +++
  virt/kvm/kvm_main.c            |  6 +++--
  7 files changed, 70 insertions(+), 34 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index bbeb0f6783d7..7c059b23ae16 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -541,12 +541,14 @@ static bool mmu_spte_update(u64 *sptep, u64 new_spte)
  
  	if (is_accessed_spte(old_spte) && !is_accessed_spte(new_spte)) {
  		flush = true;
-		kvm_set_pfn_accessed(spte_to_pfn(old_spte));
+		if (is_refcounted_page_spte(old_spte))
+			kvm_set_page_accessed(pfn_to_page(spte_to_pfn(old_spte)));
  	}
  
  	if (is_dirty_spte(old_spte) && !is_dirty_spte(new_spte)) {
  		flush = true;
-		kvm_set_pfn_dirty(spte_to_pfn(old_spte));
+		if (is_refcounted_page_spte(old_spte))
+			kvm_set_page_dirty(pfn_to_page(spte_to_pfn(old_spte)));
  	}
  
  	return flush;
@@ -578,20 +580,23 @@ static u64 mmu_spte_clear_track_bits(struct kvm *kvm, u64 *sptep)
  
  	pfn = spte_to_pfn(old_spte);
  
-	/*
-	 * KVM doesn't hold a reference to any pages mapped into the guest, and
-	 * instead uses the mmu_notifier to ensure that KVM unmaps any pages
-	 * before they are reclaimed.  Sanity check that, if the pfn is backed
-	 * by a refcounted page, the refcount is elevated.
-	 */
-	page = kvm_pfn_to_refcounted_page(pfn);
-	WARN_ON_ONCE(page && !page_count(page));
+	if (is_refcounted_page_spte(old_spte)) {
+		/*
+		 * KVM doesn't hold a reference to any pages mapped into the
+		 * guest, and instead uses the mmu_notifier to ensure that KVM
+		 * unmaps any pages before they are reclaimed. Sanity check
+		 * that, if the pfn is backed by a refcounted page, the
+		 * refcount is elevated.
+		 */
+		page = kvm_pfn_to_refcounted_page(pfn);
+		WARN_ON_ONCE(!page || !page_count(page));
  
-	if (is_accessed_spte(old_spte))
-		kvm_set_pfn_accessed(pfn);
+		if (is_accessed_spte(old_spte))
+			kvm_set_page_accessed(pfn_to_page(pfn));
  
-	if (is_dirty_spte(old_spte))
-		kvm_set_pfn_dirty(pfn);
+		if (is_dirty_spte(old_spte))
+			kvm_set_page_dirty(pfn_to_page(pfn));
+	}
  
  	return old_spte;
  }
@@ -627,8 +632,8 @@ static bool mmu_spte_age(u64 *sptep)
  		 * Capture the dirty status of the page, so that it doesn't get
  		 * lost when the SPTE is marked for access tracking.
  		 */
-		if (is_writable_pte(spte))
-			kvm_set_pfn_dirty(spte_to_pfn(spte));
+		if (is_writable_pte(spte) && is_refcounted_page_spte(spte))
+			kvm_set_page_dirty(pfn_to_page(spte_to_pfn(spte)));
  
  		spte = mark_spte_for_access_track(spte);
  		mmu_spte_update_no_track(sptep, spte);
@@ -1267,8 +1272,8 @@ static bool spte_wrprot_for_clear_dirty(u64 *sptep)
  {
  	bool was_writable = test_and_clear_bit(PT_WRITABLE_SHIFT,
  					       (unsigned long *)sptep);
-	if (was_writable && !spte_ad_enabled(*sptep))
-		kvm_set_pfn_dirty(spte_to_pfn(*sptep));
+	if (was_writable && !spte_ad_enabled(*sptep) && is_refcounted_page_spte(*sptep))
+		kvm_set_page_dirty(pfn_to_page(spte_to_pfn(*sptep)));
  
  	return was_writable;
  }
@@ -2946,7 +2951,7 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, struct kvm_memory_slot *slot,
  	}
  
  	wrprot = make_spte(vcpu, sp, slot, pte_access, gfn, pfn, *sptep, prefetch,
-			   true, host_writable, &spte);
+			   true, host_writable, true, &spte);
  
  	if (*sptep == spte) {
  		ret = RET_PF_SPURIOUS;
diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index 4d4e98fe4f35..c965f77ac4d5 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -902,7 +902,7 @@ static gpa_t FNAME(gva_to_gpa)(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
   */
  static int FNAME(sync_spte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, int i)
  {
-	bool host_writable;
+	bool host_writable, is_refcounted;
  	gpa_t first_pte_gpa;
  	u64 *sptep, spte;
  	struct kvm_memory_slot *slot;
@@ -959,10 +959,11 @@ static int FNAME(sync_spte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, int
  	sptep = &sp->spt[i];
  	spte = *sptep;
  	host_writable = spte & shadow_host_writable_mask;
+	is_refcounted = is_refcounted_page_spte(spte);
  	slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
  	make_spte(vcpu, sp, slot, pte_access, gfn,
  		  spte_to_pfn(spte), spte, true, false,
-		  host_writable, &spte);
+		  host_writable, is_refcounted, &spte);
  
  	return mmu_spte_update(sptep, spte);
  }
diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
index 4a599130e9c9..efba85df6518 100644
--- a/arch/x86/kvm/mmu/spte.c
+++ b/arch/x86/kvm/mmu/spte.c
@@ -138,7 +138,7 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
  	       const struct kvm_memory_slot *slot,
  	       unsigned int pte_access, gfn_t gfn, kvm_pfn_t pfn,
  	       u64 old_spte, bool prefetch, bool can_unsync,
-	       bool host_writable, u64 *new_spte)
+	       bool host_writable, bool is_refcounted, u64 *new_spte)
  {
  	int level = sp->role.level;
  	u64 spte = SPTE_MMU_PRESENT_MASK;
@@ -188,6 +188,8 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
  
  	if (level > PG_LEVEL_4K)
  		spte |= PT_PAGE_SIZE_MASK;
+	if (spte_has_refcount_bit() && is_refcounted)
+		spte |= SPTE_MMU_PAGE_REFCOUNTED;
  
  	if (shadow_memtype_mask)
  		spte |= static_call(kvm_x86_get_mt_mask)(vcpu, gfn,
diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
index a129951c9a88..4101cc9ef52f 100644
--- a/arch/x86/kvm/mmu/spte.h
+++ b/arch/x86/kvm/mmu/spte.h
@@ -96,6 +96,11 @@ static_assert(!(EPT_SPTE_MMU_WRITABLE & SHADOW_ACC_TRACK_SAVED_MASK));
  /* Defined only to keep the above static asserts readable. */
  #undef SHADOW_ACC_TRACK_SAVED_MASK
  
+/*
+ * Indicates that the SPTE refers to a page with a valid refcount.
+ */
+#define SPTE_MMU_PAGE_REFCOUNTED        BIT_ULL(59)
+
  /*
   * Due to limited space in PTEs, the MMIO generation is a 19 bit subset of
   * the memslots generation and is derived as follows:
@@ -345,6 +350,21 @@ static inline bool is_dirty_spte(u64 spte)
  	return dirty_mask ? spte & dirty_mask : spte & PT_WRITABLE_MASK;
  }
  
+/*
+ * Extra bits are only available for TDP SPTEs, since bits 62:52 are reserved
+ * for PAE paging, including NPT PAE. When a tracking bit isn't available, we
+ * will reject mapping non-refcounted struct pages.
+ */
+static inline bool spte_has_refcount_bit(void)
+{
+	return tdp_enabled && IS_ENABLED(CONFIG_X86_64);
+}
+
+static inline bool is_refcounted_page_spte(u64 spte)
+{
+	return !spte_has_refcount_bit() || (spte & SPTE_MMU_PAGE_REFCOUNTED);
+}
+
  static inline u64 get_rsvd_bits(struct rsvd_bits_validate *rsvd_check, u64 pte,
  				int level)
  {
@@ -475,7 +495,7 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
  	       const struct kvm_memory_slot *slot,
  	       unsigned int pte_access, gfn_t gfn, kvm_pfn_t pfn,
  	       u64 old_spte, bool prefetch, bool can_unsync,
-	       bool host_writable, u64 *new_spte);
+	       bool host_writable, bool is_refcounted, u64 *new_spte);
  u64 make_huge_page_split_spte(struct kvm *kvm, u64 huge_spte,
  		      	      union kvm_mmu_page_role role, int index);
  u64 make_nonleaf_spte(u64 *child_pt, bool ad_disabled);
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 6ae19b4ee5b1..ee497fb78d90 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -414,6 +414,7 @@ static void handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
  	bool was_leaf = was_present && is_last_spte(old_spte, level);
  	bool is_leaf = is_present && is_last_spte(new_spte, level);
  	bool pfn_changed = spte_to_pfn(old_spte) != spte_to_pfn(new_spte);
+	bool is_refcounted = is_refcounted_page_spte(old_spte);
  
  	WARN_ON_ONCE(level > PT64_ROOT_MAX_LEVEL);
  	WARN_ON_ONCE(level < PG_LEVEL_4K);
@@ -478,9 +479,9 @@ static void handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
  	if (is_leaf != was_leaf)
  		kvm_update_page_stats(kvm, level, is_leaf ? 1 : -1);
  
-	if (was_leaf && is_dirty_spte(old_spte) &&
+	if (was_leaf && is_dirty_spte(old_spte) && is_refcounted &&
  	    (!is_present || !is_dirty_spte(new_spte) || pfn_changed))
-		kvm_set_pfn_dirty(spte_to_pfn(old_spte));
+		kvm_set_page_dirty(pfn_to_page(spte_to_pfn(old_spte)));
  
  	/*
  	 * Recursively handle child PTs if the change removed a subtree from
@@ -492,9 +493,9 @@ static void handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
  	    (is_leaf || !is_present || WARN_ON_ONCE(pfn_changed)))
  		handle_removed_pt(kvm, spte_to_child_pt(old_spte, level), shared);
  
-	if (was_leaf && is_accessed_spte(old_spte) &&
+	if (was_leaf && is_accessed_spte(old_spte) && is_refcounted &&
  	    (!is_present || !is_accessed_spte(new_spte) || pfn_changed))
-		kvm_set_pfn_accessed(spte_to_pfn(old_spte));
+		kvm_set_page_accessed(pfn_to_page(spte_to_pfn(old_spte)));
  }
  
  /*
@@ -956,8 +957,8 @@ static int tdp_mmu_map_handle_target_level(struct kvm_vcpu *vcpu,
  		new_spte = make_mmio_spte(vcpu, iter->gfn, ACC_ALL);
  	else
  		wrprot = make_spte(vcpu, sp, fault->slot, ACC_ALL, iter->gfn,
-					 fault->pfn, iter->old_spte, fault->prefetch, true,
-					 fault->map_writable, &new_spte);
+				   fault->pfn, iter->old_spte, fault->prefetch, true,
+				   fault->map_writable, true, &new_spte);
  
  	if (new_spte == iter->old_spte)
  		ret = RET_PF_SPURIOUS;
@@ -1178,8 +1179,9 @@ static bool age_gfn_range(struct kvm *kvm, struct tdp_iter *iter,
  		 * Capture the dirty status of the page, so that it doesn't get
  		 * lost when the SPTE is marked for access tracking.
  		 */
-		if (is_writable_pte(iter->old_spte))
-			kvm_set_pfn_dirty(spte_to_pfn(iter->old_spte));
+		if (is_writable_pte(iter->old_spte) &&
+		    is_refcounted_page_spte(iter->old_spte))
+			kvm_set_page_dirty(pfn_to_page(spte_to_pfn(iter->old_spte)));
  
  		new_spte = mark_spte_for_access_track(iter->old_spte);
  		iter->old_spte = kvm_tdp_mmu_write_spte(iter->sptep,
@@ -1602,7 +1604,8 @@ static void clear_dirty_pt_masked(struct kvm *kvm, struct kvm_mmu_page *root,
  		trace_kvm_tdp_mmu_spte_changed(iter.as_id, iter.gfn, iter.level,
  					       iter.old_spte,
  					       iter.old_spte & ~dbit);
-		kvm_set_pfn_dirty(spte_to_pfn(iter.old_spte));
+		if (is_refcounted_page_spte(iter.old_spte))
+			kvm_set_page_dirty(pfn_to_page(spte_to_pfn(iter.old_spte)));
  	}
  
  	rcu_read_unlock();
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index f72c79f159a2..cff5df6b0c52 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1211,6 +1211,9 @@ unsigned long gfn_to_hva_memslot_prot(struct kvm_memory_slot *slot, gfn_t gfn,
  void kvm_release_page_clean(struct page *page);
  void kvm_release_page_dirty(struct page *page);
  
+void kvm_set_page_accessed(struct page *page);
+void kvm_set_page_dirty(struct page *page);
+
  struct kvm_follow_pfn {
  	const struct kvm_memory_slot *slot;
  	gfn_t gfn;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 5d66d841e775..e53a14adf149 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3281,17 +3281,19 @@ static bool kvm_is_ad_tracked_page(struct page *page)
  	return !PageReserved(page);
  }
  
-static void kvm_set_page_dirty(struct page *page)
+void kvm_set_page_dirty(struct page *page)
  {
  	if (kvm_is_ad_tracked_page(page))
  		SetPageDirty(page);
  }
+EXPORT_SYMBOL_GPL(kvm_set_page_dirty);
  
-static void kvm_set_page_accessed(struct page *page)
+void kvm_set_page_accessed(struct page *page)
  {
  	if (kvm_is_ad_tracked_page(page))
  		mark_page_accessed(page);
  }
+EXPORT_SYMBOL_GPL(kvm_set_page_accessed);
  
  void kvm_release_page_clean(struct page *page)
  {








