[PATCH v6 0/8] KVM: x86: Apply NX mitigation more precisely

[PATCH v6 0/8] KVM: x86: Apply NX mitigation more precisely

[Sean Christopherson]

David and Ben, I kept your Reviewed-by for patch 7 even though it's now a
fairly different patch than what you reviewed, as the code generation is
identical.  Holler if you want your review dropped.

Precisely track (via kvm_mmu_page) if a non-huge page is being forced
and use that info to avoid unnecessarily forcing smaller page sizes in
disallowed_hugepage_adjust().

KVM incorrectly assumes that the NX huge page mitigation is the only
scenario where KVM will create a non-leaf page instead of a huge page.
As a result, if the original source of huge page incompatibility goes
away, the NX mitigation is enabled, and KVM encounters an present shadow
page when attempting to install a huge page, KVM will force a smaller page
regardless of whether or not a smaller page is actually necessary to
satisfy the NX huge page mitigation.

Unnecessarily forcing small pages can result in degraded guest performance,
especially on larger VMs.  The bug was originally discovered when testing
dirty log performance, as KVM would leave small pages lying around when
zapping collapsible SPTEs.  That case was indadvertantly fixed by commit
5ba7c4c6d1c7 ("KVM: x86/MMU: Zap non-leaf SPTEs when disabling dirty
logging"), but other scenarios are still affected, e.g. KVM will not
rebuild a huge page if the mmu_notifier zaps a range of PTEs because the
primary MMU is creating a huge page.

v6:
 - Fold the nx_huge_page_disallowed check in disallowed_hugepage_adjust()
   into the existing if-statement. [Yan]
 - Add a patch to extend the accounting sanity check in recovery to the
   TDP MMU. [Yan]

v5:
 - Drop boneheaded KVM_BUG_ON() GFN aliasing. [Vitaly]
 - Drop incorrect barrier documentation. [Yan]

v4:
 - https://lore.kernel.org/all/20220830235537.4004585-1-seanjc@google.com
 - Collect reviews. [Mingwei]
 - Add comment to document possible_nx_huge_pages. [Mingwei]
 - Drop extra memory barriers. [Paolo]
 - Document ordering providing by TDP SPTE helpers. [Paolo]

v3:
 - https://lore.kernel.org/all/20220805230513.148869-1-seanjc@google.com
 - Bug the VM if KVM attempts to double account a shadow page that
   disallows a NX huge page. [David]
 - Split the rename to separate patch. [Paolo]
 - Rename more NX huge page variables/functions. [David]
 - Combine and tweak the comments about enforcing the NX huge page
   mitigation for non-paging MMUs. [Paolo, David]
 - Call out that the shadow MMU holds mmu_lock for write and doesn't need
   to manual handle memory ordering when accounting NX huge pages. [David]
 - Add a smp_rmb() when unlinking shadow pages in the TDP MMU.
 - Rename spte_to_sp() to spte_to_child_sp(). [David]
 - Collect reviews. [David]
 - Tweak the changelog for the final patch to call out that precise
   accounting addresses real world performance bugs. [Paolo]
 - Reword the changelog for the patch to (almost) always tag disallowed
   NX huge pages, and call out that it doesn't fix the TDP MMU. [David]

v2: Rebase, tweak a changelog accordingly.

v1: https://lore.kernel.org/all/20220409003847.819686-1-seanjc@google.com


Mingwei Zhang (1):
  KVM: x86/mmu: explicitly check nx_hugepage in
    disallowed_hugepage_adjust()

Sean Christopherson (7):
  KVM: x86/mmu: Tag disallowed NX huge pages even if they're not tracked
  KVM: x86/mmu: Rename NX huge pages fields/functions for consistency
  KVM: x86/mmu: Properly account NX huge page workaround for nonpaging
    MMUs
  KVM: x86/mmu: Set disallowed_nx_huge_page in TDP MMU before setting
    SPTE
  KVM: x86/mmu: Track the number of TDP MMU pages, but not the actual
    pages
  KVM: x86/mmu: Add helper to convert SPTE value to its shadow page
  KVM: x86/mmu: WARN if TDP MMU SP disallows hugepage after being zapped

 arch/x86/include/asm/kvm_host.h |  30 ++++----
 arch/x86/kvm/mmu/mmu.c          | 122 ++++++++++++++++++++------------
 arch/x86/kvm/mmu/mmu_internal.h |  33 ++++-----
 arch/x86/kvm/mmu/paging_tmpl.h  |   6 +-
 arch/x86/kvm/mmu/spte.c         |  12 ++++
 arch/x86/kvm/mmu/spte.h         |  17 +++++
 arch/x86/kvm/mmu/tdp_mmu.c      |  43 ++++++-----
 arch/x86/kvm/mmu/tdp_mmu.h      |   2 +
 8 files changed, 170 insertions(+), 95 deletions(-)


base-commit: e18d6152ff0f41b7f01f9817372022df04e0d354
-- 
2.38.0.413.g74048e4d9e-goog

[PATCH v6 1/8] KVM: x86/mmu: Tag disallowed NX huge pages even if they're not tracked

[Sean Christopherson]

Tag shadow pages that cannot be replaced with an NX huge page regardless
of whether or not zapping the page would allow KVM to immediately create
a huge page, e.g. because something else prevents creating a huge page.

I.e. track pages that are disallowed from being NX huge pages regardless
of whether or not the page could have been huge at the time of fault.
KVM currently tracks pages that were disallowed from being huge due to
the NX workaround if and only if the page could otherwise be huge.  But
that fails to handled the scenario where whatever restriction prevented
KVM from installing a huge page goes away, e.g. if dirty logging is
disabled, the host mapping level changes, etc...

Failure to tag shadow pages appropriately could theoretically lead to
false negatives, e.g. if a fetch fault requests a small page and thus
isn't tracked, and a read/write fault later requests a huge page, KVM
will not reject the huge page as it should.

To avoid yet another flag, initialize the list_head and use list_empty()
to determine whether or not a page is on the list of NX huge pages that
should be recovered.

Note, the TDP MMU accounting is still flawed as fixing the TDP MMU is
more involved due to mmu_lock being held for read.  This will be
addressed in a future commit.

Fixes: 5bcaf3e1715f ("KVM: x86/mmu: Account NX huge page disallowed iff huge page was requested")
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/mmu/mmu.c          | 32 ++++++++++++++++++++++++--------
 arch/x86/kvm/mmu/mmu_internal.h | 10 +++++++++-
 arch/x86/kvm/mmu/paging_tmpl.h  |  6 +++---
 arch/x86/kvm/mmu/tdp_mmu.c      |  4 +++-
 4 files changed, 39 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 6f81539061d6..f1e089dfdd22 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -802,15 +802,25 @@ static void account_shadowed(struct kvm *kvm, struct kvm_mmu_page *sp)
 		kvm_flush_remote_tlbs_with_address(kvm, gfn, 1);
 }
 
-void account_huge_nx_page(struct kvm *kvm, struct kvm_mmu_page *sp)
+void account_huge_nx_page(struct kvm *kvm, struct kvm_mmu_page *sp,
+			  bool nx_huge_page_possible)
 {
-	if (sp->lpage_disallowed)
+	sp->lpage_disallowed = true;
+
+	/*
+	 * If it's possible to replace the shadow page with an NX huge page,
+	 * i.e. if the shadow page is the only thing currently preventing KVM
+	 * from using a huge page, add the shadow page to the list of "to be
+	 * zapped for NX recovery" pages.  Note, the shadow page can already be
+	 * on the list if KVM is reusing an existing shadow page, i.e. if KVM
+	 * links a shadow page at multiple points.
+	 */
+	if (!nx_huge_page_possible || !list_empty(&sp->lpage_disallowed_link))
 		return;
 
 	++kvm->stat.nx_lpage_splits;
 	list_add_tail(&sp->lpage_disallowed_link,
 		      &kvm->arch.lpage_disallowed_mmu_pages);
-	sp->lpage_disallowed = true;
 }
 
 static void unaccount_shadowed(struct kvm *kvm, struct kvm_mmu_page *sp)
@@ -832,9 +842,13 @@ static void unaccount_shadowed(struct kvm *kvm, struct kvm_mmu_page *sp)
 
 void unaccount_huge_nx_page(struct kvm *kvm, struct kvm_mmu_page *sp)
 {
-	--kvm->stat.nx_lpage_splits;
 	sp->lpage_disallowed = false;
-	list_del(&sp->lpage_disallowed_link);
+
+	if (list_empty(&sp->lpage_disallowed_link))
+		return;
+
+	--kvm->stat.nx_lpage_splits;
+	list_del_init(&sp->lpage_disallowed_link);
 }
 
 static struct kvm_memory_slot *
@@ -2129,6 +2143,8 @@ static struct kvm_mmu_page *kvm_mmu_alloc_shadow_page(struct kvm *kvm,
 
 	set_page_private(virt_to_page(sp->spt), (unsigned long)sp);
 
+	INIT_LIST_HEAD(&sp->lpage_disallowed_link);
+
 	/*
 	 * active_mmu_pages must be a FIFO list, as kvm_zap_obsolete_pages()
 	 * depends on valid pages being added to the head of the list.  See
@@ -3126,9 +3142,9 @@ static int __direct_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 			continue;
 
 		link_shadow_page(vcpu, it.sptep, sp);
-		if (fault->is_tdp && fault->huge_page_disallowed &&
-		    fault->req_level >= it.level)
-			account_huge_nx_page(vcpu->kvm, sp);
+		if (fault->is_tdp && fault->huge_page_disallowed)
+			account_huge_nx_page(vcpu->kvm, sp,
+					     fault->req_level >= it.level);
 	}
 
 	if (WARN_ON_ONCE(it.level != fault->goal_level))
diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
index 582def531d4d..cca1ad75d096 100644
--- a/arch/x86/kvm/mmu/mmu_internal.h
+++ b/arch/x86/kvm/mmu/mmu_internal.h
@@ -100,6 +100,13 @@ struct kvm_mmu_page {
 		};
 	};
 
+	/*
+	 * Tracks shadow pages that, if zapped, would allow KVM to create an NX
+	 * huge page.  A shadow page will have lpage_disallowed set but not be
+	 * on the list if a huge page is disallowed for other reasons, e.g.
+	 * because KVM is shadowing a PTE at the same gfn, the memslot isn't
+	 * properly aligned, etc...
+	 */
 	struct list_head lpage_disallowed_link;
 #ifdef CONFIG_X86_32
 	/*
@@ -315,7 +322,8 @@ void disallowed_hugepage_adjust(struct kvm_page_fault *fault, u64 spte, int cur_
 
 void *mmu_memory_cache_alloc(struct kvm_mmu_memory_cache *mc);
 
-void account_huge_nx_page(struct kvm *kvm, struct kvm_mmu_page *sp);
+void account_huge_nx_page(struct kvm *kvm, struct kvm_mmu_page *sp,
+			  bool nx_huge_page_possible);
 void unaccount_huge_nx_page(struct kvm *kvm, struct kvm_mmu_page *sp);
 
 #endif /* __KVM_X86_MMU_INTERNAL_H */
diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index 5ab5f94dcb6f..8fd0c4e1e575 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -713,9 +713,9 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
 			continue;
 
 		link_shadow_page(vcpu, it.sptep, sp);
-		if (fault->huge_page_disallowed &&
-		    fault->req_level >= it.level)
-			account_huge_nx_page(vcpu->kvm, sp);
+		if (fault->huge_page_disallowed)
+			account_huge_nx_page(vcpu->kvm, sp,
+					     fault->req_level >= it.level);
 	}
 
 	if (WARN_ON_ONCE(it.level != fault->goal_level))
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 672f0432d777..80a4a1a09131 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -284,6 +284,8 @@ static struct kvm_mmu_page *tdp_mmu_alloc_sp(struct kvm_vcpu *vcpu)
 static void tdp_mmu_init_sp(struct kvm_mmu_page *sp, tdp_ptep_t sptep,
 			    gfn_t gfn, union kvm_mmu_page_role role)
 {
+	INIT_LIST_HEAD(&sp->lpage_disallowed_link);
+
 	set_page_private(virt_to_page(sp->spt), (unsigned long)sp);
 
 	sp->role = role;
@@ -1141,7 +1143,7 @@ static int tdp_mmu_link_sp(struct kvm *kvm, struct tdp_iter *iter,
 	spin_lock(&kvm->arch.tdp_mmu_pages_lock);
 	list_add(&sp->link, &kvm->arch.tdp_mmu_pages);
 	if (account_nx)
-		account_huge_nx_page(kvm, sp);
+		account_huge_nx_page(kvm, sp, true);
 	spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
 	tdp_account_mmu_page(kvm, sp);
 
-- 
2.38.0.413.g74048e4d9e-goog

[PATCH v6 2/8] KVM: x86/mmu: Rename NX huge pages fields/functions for consistency

[Sean Christopherson]

Rename most of the variables/functions involved in the NX huge page
mitigation to provide consistency, e.g. lpage vs huge page, and NX huge
vs huge NX, and also to provide clarity, e.g. to make it obvious the flag
applies only to the NX huge page mitigation, not to any condition that
prevents creating a huge page.

Add a comment explaining what the newly named "possible_nx_huge_pages"
tracks.

Leave the nx_lpage_splits stat alone as the name is ABI and thus set in
stone.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Mingwei Zhang <mizhang@google.com>
---
 arch/x86/include/asm/kvm_host.h | 19 +++++++--
 arch/x86/kvm/mmu/mmu.c          | 71 +++++++++++++++++----------------
 arch/x86/kvm/mmu/mmu_internal.h | 22 ++++++----
 arch/x86/kvm/mmu/paging_tmpl.h  |  2 +-
 arch/x86/kvm/mmu/tdp_mmu.c      |  8 ++--
 5 files changed, 71 insertions(+), 51 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 7551b6f9c31c..0333dbb8ec85 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1151,7 +1151,18 @@ struct kvm_arch {
 	struct hlist_head mmu_page_hash[KVM_NUM_MMU_PAGES];
 	struct list_head active_mmu_pages;
 	struct list_head zapped_obsolete_pages;
-	struct list_head lpage_disallowed_mmu_pages;
+	/*
+	 * A list of kvm_mmu_page structs that, if zapped, could possibly be
+	 * replaced by an NX huge page.  A shadow page is on this list if its
+	 * existence disallows an NX huge page (nx_huge_page_disallowed is set)
+	 * and there are no other conditions that prevent a huge page, e.g.
+	 * the backing host page is huge, dirtly logging is not enabled for its
+	 * memslot, etc...  Note, zapping shadow pages on this list doesn't
+	 * guarantee an NX huge page will be created in its stead, e.g. if the
+	 * guest attempts to execute from the region then KVM obviously can't
+	 * create an NX huge page (without hanging the guest).
+	 */
+	struct list_head possible_nx_huge_pages;
 	struct kvm_page_track_notifier_node mmu_sp_tracker;
 	struct kvm_page_track_notifier_head track_notifier_head;
 	/*
@@ -1267,7 +1278,7 @@ struct kvm_arch {
 	bool sgx_provisioning_allowed;
 
 	struct kvm_pmu_event_filter __rcu *pmu_event_filter;
-	struct task_struct *nx_lpage_recovery_thread;
+	struct task_struct *nx_huge_page_recovery_thread;
 
 #ifdef CONFIG_X86_64
 	/*
@@ -1312,8 +1323,8 @@ struct kvm_arch {
 	 *  - tdp_mmu_roots (above)
 	 *  - tdp_mmu_pages (above)
 	 *  - the link field of kvm_mmu_page structs used by the TDP MMU
-	 *  - lpage_disallowed_mmu_pages
-	 *  - the lpage_disallowed_link field of kvm_mmu_page structs used
+	 *  - possible_nx_huge_pages;
+	 *  - the possible_nx_huge_page_link field of kvm_mmu_page structs used
 	 *    by the TDP MMU
 	 * It is acceptable, but not necessary, to acquire this lock when
 	 * the thread holds the MMU lock in write mode.
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index f1e089dfdd22..5dd98cdc5283 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -802,10 +802,10 @@ static void account_shadowed(struct kvm *kvm, struct kvm_mmu_page *sp)
 		kvm_flush_remote_tlbs_with_address(kvm, gfn, 1);
 }
 
-void account_huge_nx_page(struct kvm *kvm, struct kvm_mmu_page *sp,
+void account_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp,
 			  bool nx_huge_page_possible)
 {
-	sp->lpage_disallowed = true;
+	sp->nx_huge_page_disallowed = true;
 
 	/*
 	 * If it's possible to replace the shadow page with an NX huge page,
@@ -815,12 +815,13 @@ void account_huge_nx_page(struct kvm *kvm, struct kvm_mmu_page *sp,
 	 * on the list if KVM is reusing an existing shadow page, i.e. if KVM
 	 * links a shadow page at multiple points.
 	 */
-	if (!nx_huge_page_possible || !list_empty(&sp->lpage_disallowed_link))
+	if (!nx_huge_page_possible ||
+	    !list_empty(&sp->possible_nx_huge_page_link))
 		return;
 
 	++kvm->stat.nx_lpage_splits;
-	list_add_tail(&sp->lpage_disallowed_link,
-		      &kvm->arch.lpage_disallowed_mmu_pages);
+	list_add_tail(&sp->possible_nx_huge_page_link,
+		      &kvm->arch.possible_nx_huge_pages);
 }
 
 static void unaccount_shadowed(struct kvm *kvm, struct kvm_mmu_page *sp)
@@ -840,15 +841,15 @@ static void unaccount_shadowed(struct kvm *kvm, struct kvm_mmu_page *sp)
 	kvm_mmu_gfn_allow_lpage(slot, gfn);
 }
 
-void unaccount_huge_nx_page(struct kvm *kvm, struct kvm_mmu_page *sp)
+void unaccount_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp)
 {
-	sp->lpage_disallowed = false;
+	sp->nx_huge_page_disallowed = false;
 
-	if (list_empty(&sp->lpage_disallowed_link))
+	if (list_empty(&sp->possible_nx_huge_page_link))
 		return;
 
 	--kvm->stat.nx_lpage_splits;
-	list_del_init(&sp->lpage_disallowed_link);
+	list_del_init(&sp->possible_nx_huge_page_link);
 }
 
 static struct kvm_memory_slot *
@@ -2143,7 +2144,7 @@ static struct kvm_mmu_page *kvm_mmu_alloc_shadow_page(struct kvm *kvm,
 
 	set_page_private(virt_to_page(sp->spt), (unsigned long)sp);
 
-	INIT_LIST_HEAD(&sp->lpage_disallowed_link);
+	INIT_LIST_HEAD(&sp->possible_nx_huge_page_link);
 
 	/*
 	 * active_mmu_pages must be a FIFO list, as kvm_zap_obsolete_pages()
@@ -2502,8 +2503,8 @@ static bool __kvm_mmu_prepare_zap_page(struct kvm *kvm,
 		zapped_root = !is_obsolete_sp(kvm, sp);
 	}
 
-	if (sp->lpage_disallowed)
-		unaccount_huge_nx_page(kvm, sp);
+	if (sp->nx_huge_page_disallowed)
+		unaccount_nx_huge_page(kvm, sp);
 
 	sp->role.invalid = 1;
 
@@ -3143,7 +3144,7 @@ static int __direct_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 
 		link_shadow_page(vcpu, it.sptep, sp);
 		if (fault->is_tdp && fault->huge_page_disallowed)
-			account_huge_nx_page(vcpu->kvm, sp,
+			account_nx_huge_page(vcpu->kvm, sp,
 					     fault->req_level >= it.level);
 	}
 
@@ -5987,7 +5988,7 @@ int kvm_mmu_init_vm(struct kvm *kvm)
 
 	INIT_LIST_HEAD(&kvm->arch.active_mmu_pages);
 	INIT_LIST_HEAD(&kvm->arch.zapped_obsolete_pages);
-	INIT_LIST_HEAD(&kvm->arch.lpage_disallowed_mmu_pages);
+	INIT_LIST_HEAD(&kvm->arch.possible_nx_huge_pages);
 	spin_lock_init(&kvm->arch.mmu_unsync_pages_lock);
 
 	r = kvm_mmu_init_tdp_mmu(kvm);
@@ -6672,7 +6673,7 @@ static int set_nx_huge_pages(const char *val, const struct kernel_param *kp)
 			kvm_mmu_zap_all_fast(kvm);
 			mutex_unlock(&kvm->slots_lock);
 
-			wake_up_process(kvm->arch.nx_lpage_recovery_thread);
+			wake_up_process(kvm->arch.nx_huge_page_recovery_thread);
 		}
 		mutex_unlock(&kvm_lock);
 	}
@@ -6804,7 +6805,7 @@ static int set_nx_huge_pages_recovery_param(const char *val, const struct kernel
 		mutex_lock(&kvm_lock);
 
 		list_for_each_entry(kvm, &vm_list, vm_list)
-			wake_up_process(kvm->arch.nx_lpage_recovery_thread);
+			wake_up_process(kvm->arch.nx_huge_page_recovery_thread);
 
 		mutex_unlock(&kvm_lock);
 	}
@@ -6812,7 +6813,7 @@ static int set_nx_huge_pages_recovery_param(const char *val, const struct kernel
 	return err;
 }
 
-static void kvm_recover_nx_lpages(struct kvm *kvm)
+static void kvm_recover_nx_huge_pages(struct kvm *kvm)
 {
 	unsigned long nx_lpage_splits = kvm->stat.nx_lpage_splits;
 	int rcu_idx;
@@ -6835,23 +6836,25 @@ static void kvm_recover_nx_lpages(struct kvm *kvm)
 	ratio = READ_ONCE(nx_huge_pages_recovery_ratio);
 	to_zap = ratio ? DIV_ROUND_UP(nx_lpage_splits, ratio) : 0;
 	for ( ; to_zap; --to_zap) {
-		if (list_empty(&kvm->arch.lpage_disallowed_mmu_pages))
+		if (list_empty(&kvm->arch.possible_nx_huge_pages))
 			break;
 
 		/*
 		 * We use a separate list instead of just using active_mmu_pages
-		 * because the number of lpage_disallowed pages is expected to
-		 * be relatively small compared to the total.
+		 * because the number of shadow pages that be replaced with an
+		 * NX huge page is expected to be relatively small compared to
+		 * the total number of shadow pages.  And because the TDP MMU
+		 * doesn't use active_mmu_pages.
 		 */
-		sp = list_first_entry(&kvm->arch.lpage_disallowed_mmu_pages,
+		sp = list_first_entry(&kvm->arch.possible_nx_huge_pages,
 				      struct kvm_mmu_page,
-				      lpage_disallowed_link);
-		WARN_ON_ONCE(!sp->lpage_disallowed);
+				      possible_nx_huge_page_link);
+		WARN_ON_ONCE(!sp->nx_huge_page_disallowed);
 		if (is_tdp_mmu_page(sp)) {
 			flush |= kvm_tdp_mmu_zap_sp(kvm, sp);
 		} else {
 			kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list);
-			WARN_ON_ONCE(sp->lpage_disallowed);
+			WARN_ON_ONCE(sp->nx_huge_page_disallowed);
 		}
 
 		if (need_resched() || rwlock_needbreak(&kvm->mmu_lock)) {
@@ -6872,7 +6875,7 @@ static void kvm_recover_nx_lpages(struct kvm *kvm)
 	srcu_read_unlock(&kvm->srcu, rcu_idx);
 }
 
-static long get_nx_lpage_recovery_timeout(u64 start_time)
+static long get_nx_huge_page_recovery_timeout(u64 start_time)
 {
 	bool enabled;
 	uint period;
@@ -6883,19 +6886,19 @@ static long get_nx_lpage_recovery_timeout(u64 start_time)
 		       : MAX_SCHEDULE_TIMEOUT;
 }
 
-static int kvm_nx_lpage_recovery_worker(struct kvm *kvm, uintptr_t data)
+static int kvm_nx_huge_page_recovery_worker(struct kvm *kvm, uintptr_t data)
 {
 	u64 start_time;
 	long remaining_time;
 
 	while (true) {
 		start_time = get_jiffies_64();
-		remaining_time = get_nx_lpage_recovery_timeout(start_time);
+		remaining_time = get_nx_huge_page_recovery_timeout(start_time);
 
 		set_current_state(TASK_INTERRUPTIBLE);
 		while (!kthread_should_stop() && remaining_time > 0) {
 			schedule_timeout(remaining_time);
-			remaining_time = get_nx_lpage_recovery_timeout(start_time);
+			remaining_time = get_nx_huge_page_recovery_timeout(start_time);
 			set_current_state(TASK_INTERRUPTIBLE);
 		}
 
@@ -6904,7 +6907,7 @@ static int kvm_nx_lpage_recovery_worker(struct kvm *kvm, uintptr_t data)
 		if (kthread_should_stop())
 			return 0;
 
-		kvm_recover_nx_lpages(kvm);
+		kvm_recover_nx_huge_pages(kvm);
 	}
 }
 
@@ -6912,17 +6915,17 @@ int kvm_mmu_post_init_vm(struct kvm *kvm)
 {
 	int err;
 
-	err = kvm_vm_create_worker_thread(kvm, kvm_nx_lpage_recovery_worker, 0,
+	err = kvm_vm_create_worker_thread(kvm, kvm_nx_huge_page_recovery_worker, 0,
 					  "kvm-nx-lpage-recovery",
-					  &kvm->arch.nx_lpage_recovery_thread);
+					  &kvm->arch.nx_huge_page_recovery_thread);
 	if (!err)
-		kthread_unpark(kvm->arch.nx_lpage_recovery_thread);
+		kthread_unpark(kvm->arch.nx_huge_page_recovery_thread);
 
 	return err;
 }
 
 void kvm_mmu_pre_destroy_vm(struct kvm *kvm)
 {
-	if (kvm->arch.nx_lpage_recovery_thread)
-		kthread_stop(kvm->arch.nx_lpage_recovery_thread);
+	if (kvm->arch.nx_huge_page_recovery_thread)
+		kthread_stop(kvm->arch.nx_huge_page_recovery_thread);
 }
diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
index cca1ad75d096..67879459a25c 100644
--- a/arch/x86/kvm/mmu/mmu_internal.h
+++ b/arch/x86/kvm/mmu/mmu_internal.h
@@ -57,7 +57,13 @@ struct kvm_mmu_page {
 	bool tdp_mmu_page;
 	bool unsync;
 	u8 mmu_valid_gen;
-	bool lpage_disallowed; /* Can't be replaced by an equiv large page */
+
+	 /*
+	  * The shadow page can't be replaced by an equivalent huge page
+	  * because it is being used to map an executable page in the guest
+	  * and the NX huge page mitigation is enabled.
+	  */
+	bool nx_huge_page_disallowed;
 
 	/*
 	 * The following two entries are used to key the shadow page in the
@@ -102,12 +108,12 @@ struct kvm_mmu_page {
 
 	/*
 	 * Tracks shadow pages that, if zapped, would allow KVM to create an NX
-	 * huge page.  A shadow page will have lpage_disallowed set but not be
-	 * on the list if a huge page is disallowed for other reasons, e.g.
-	 * because KVM is shadowing a PTE at the same gfn, the memslot isn't
-	 * properly aligned, etc...
+	 * huge page.  A shadow page will have nx_huge_page_disallowed set but
+	 * not be on the list if a huge page is disallowed for other reasons,
+	 * e.g. because KVM is shadowing a PTE at the same gfn, the memslot
+	 * isn't properly aligned, etc...
 	 */
-	struct list_head lpage_disallowed_link;
+	struct list_head possible_nx_huge_page_link;
 #ifdef CONFIG_X86_32
 	/*
 	 * Used out of the mmu-lock to avoid reading spte values while an
@@ -322,8 +328,8 @@ void disallowed_hugepage_adjust(struct kvm_page_fault *fault, u64 spte, int cur_
 
 void *mmu_memory_cache_alloc(struct kvm_mmu_memory_cache *mc);
 
-void account_huge_nx_page(struct kvm *kvm, struct kvm_mmu_page *sp,
+void account_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp,
 			  bool nx_huge_page_possible);
-void unaccount_huge_nx_page(struct kvm *kvm, struct kvm_mmu_page *sp);
+void unaccount_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp);
 
 #endif /* __KVM_X86_MMU_INTERNAL_H */
diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index 8fd0c4e1e575..0f6455072055 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -714,7 +714,7 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
 
 		link_shadow_page(vcpu, it.sptep, sp);
 		if (fault->huge_page_disallowed)
-			account_huge_nx_page(vcpu->kvm, sp,
+			account_nx_huge_page(vcpu->kvm, sp,
 					     fault->req_level >= it.level);
 	}
 
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 80a4a1a09131..73eb28ed1f03 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -284,7 +284,7 @@ static struct kvm_mmu_page *tdp_mmu_alloc_sp(struct kvm_vcpu *vcpu)
 static void tdp_mmu_init_sp(struct kvm_mmu_page *sp, tdp_ptep_t sptep,
 			    gfn_t gfn, union kvm_mmu_page_role role)
 {
-	INIT_LIST_HEAD(&sp->lpage_disallowed_link);
+	INIT_LIST_HEAD(&sp->possible_nx_huge_page_link);
 
 	set_page_private(virt_to_page(sp->spt), (unsigned long)sp);
 
@@ -403,8 +403,8 @@ static void tdp_mmu_unlink_sp(struct kvm *kvm, struct kvm_mmu_page *sp,
 		lockdep_assert_held_write(&kvm->mmu_lock);
 
 	list_del(&sp->link);
-	if (sp->lpage_disallowed)
-		unaccount_huge_nx_page(kvm, sp);
+	if (sp->nx_huge_page_disallowed)
+		unaccount_nx_huge_page(kvm, sp);
 
 	if (shared)
 		spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
@@ -1143,7 +1143,7 @@ static int tdp_mmu_link_sp(struct kvm *kvm, struct tdp_iter *iter,
 	spin_lock(&kvm->arch.tdp_mmu_pages_lock);
 	list_add(&sp->link, &kvm->arch.tdp_mmu_pages);
 	if (account_nx)
-		account_huge_nx_page(kvm, sp, true);
+		account_nx_huge_page(kvm, sp, true);
 	spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
 	tdp_account_mmu_page(kvm, sp);
 
-- 
2.38.0.413.g74048e4d9e-goog

[PATCH v6 3/8] KVM: x86/mmu: Properly account NX huge page workaround for nonpaging MMUs

[Sean Christopherson]

Account and track NX huge pages for nonpaging MMUs so that a future
enhancement to precisely check if a shadow page can't be replaced by a NX
huge page doesn't get false positives.  Without correct tracking, KVM can
get stuck in a loop if an instruction is fetching and writing data on the
same huge page, e.g. KVM installs a small executable page on the fetch
fault, replaces it with an NX huge page on the write fault, and faults
again on the fetch.

Alternatively, and perhaps ideally, KVM would simply not enforce the
workaround for nonpaging MMUs.  The guest has no page tables to abuse
and KVM is guaranteed to switch to a different MMU on CR0.PG being
toggled so there's no security or performance concerns.  However, getting
make_spte() to play nice now and in the future is unnecessarily complex.

In the current code base, make_spte() can enforce the mitigation if TDP
is enabled or the MMU is indirect, but make_spte() may not always have a
vCPU/MMU to work with, e.g. if KVM were to support in-line huge page
promotion when disabling dirty logging.

Without a vCPU/MMU, KVM could either pass in the correct information
and/or derive it from the shadow page, but the former is ugly and the
latter subtly non-trivial due to the possibility of direct shadow pages
in indirect MMUs.  Given that using shadow paging with an unpaged guest
is far from top priority _and_ has been subjected to the workaround since
its inception, keep it simple and just fix the accounting glitch.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: David Matlack <dmatlack@google.com>
Reviewed-by: Mingwei Zhang <mizhang@google.com>
---
 arch/x86/kvm/mmu/mmu.c  |  2 +-
 arch/x86/kvm/mmu/spte.c | 12 ++++++++++++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 5dd98cdc5283..99086a684dd2 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3143,7 +3143,7 @@ static int __direct_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 			continue;
 
 		link_shadow_page(vcpu, it.sptep, sp);
-		if (fault->is_tdp && fault->huge_page_disallowed)
+		if (fault->huge_page_disallowed)
 			account_nx_huge_page(vcpu->kvm, sp,
 					     fault->req_level >= it.level);
 	}
diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
index 2e08b2a45361..c0fd7e049b4e 100644
--- a/arch/x86/kvm/mmu/spte.c
+++ b/arch/x86/kvm/mmu/spte.c
@@ -161,6 +161,18 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
 	if (!prefetch)
 		spte |= spte_shadow_accessed_mask(spte);
 
+	/*
+	 * For simplicity, enforce the NX huge page mitigation even if not
+	 * strictly necessary.  KVM could ignore the mitigation if paging is
+	 * disabled in the guest, as the guest doesn't have an page tables to
+	 * abuse.  But to safely ignore the mitigation, KVM would have to
+	 * ensure a new MMU is loaded (or all shadow pages zapped) when CR0.PG
+	 * is toggled on, and that's a net negative for performance when TDP is
+	 * enabled.  When TDP is disabled, KVM will always switch to a new MMU
+	 * when CR0.PG is toggled, but leveraging that to ignore the mitigation
+	 * would tie make_spte() further to vCPU/MMU state, and add complexity
+	 * just to optimize a mode that is anything but performance critical.
+	 */
 	if (level > PG_LEVEL_4K && (pte_access & ACC_EXEC_MASK) &&
 	    is_nx_huge_page_enabled(vcpu->kvm)) {
 		pte_access &= ~ACC_EXEC_MASK;
-- 
2.38.0.413.g74048e4d9e-goog

[PATCH v6 4/8] KVM: x86/mmu: Set disallowed_nx_huge_page in TDP MMU before setting SPTE

[Sean Christopherson]

Set nx_huge_page_disallowed in TDP MMU shadow pages before making the SP
visible to other readers, i.e. before setting its SPTE.  This will allow
KVM to query the flag when determining if a shadow page can be replaced
by a NX huge page without violating the rules of the mitigation.

Note, the shadow/legacy MMU holds mmu_lock for write, so it's impossible
for another CPU to see a shadow page without an up-to-date
nx_huge_page_disallowed, i.e. only the TDP MMU needs the complicated
dance.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: David Matlack <dmatlack@google.com>
---
 arch/x86/kvm/mmu/mmu.c          | 28 +++++++++++++++++++---------
 arch/x86/kvm/mmu/mmu_internal.h |  5 ++---
 arch/x86/kvm/mmu/tdp_mmu.c      | 31 ++++++++++++++++++-------------
 3 files changed, 39 insertions(+), 25 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 99086a684dd2..57c7c52d137a 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -802,11 +802,8 @@ static void account_shadowed(struct kvm *kvm, struct kvm_mmu_page *sp)
 		kvm_flush_remote_tlbs_with_address(kvm, gfn, 1);
 }
 
-void account_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp,
-			  bool nx_huge_page_possible)
+void track_possible_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp)
 {
-	sp->nx_huge_page_disallowed = true;
-
 	/*
 	 * If it's possible to replace the shadow page with an NX huge page,
 	 * i.e. if the shadow page is the only thing currently preventing KVM
@@ -815,8 +812,7 @@ void account_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp,
 	 * on the list if KVM is reusing an existing shadow page, i.e. if KVM
 	 * links a shadow page at multiple points.
 	 */
-	if (!nx_huge_page_possible ||
-	    !list_empty(&sp->possible_nx_huge_page_link))
+	if (!list_empty(&sp->possible_nx_huge_page_link))
 		return;
 
 	++kvm->stat.nx_lpage_splits;
@@ -824,6 +820,15 @@ void account_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp,
 		      &kvm->arch.possible_nx_huge_pages);
 }
 
+static void account_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp,
+				 bool nx_huge_page_possible)
+{
+	sp->nx_huge_page_disallowed = true;
+
+	if (nx_huge_page_possible)
+		track_possible_nx_huge_page(kvm, sp);
+}
+
 static void unaccount_shadowed(struct kvm *kvm, struct kvm_mmu_page *sp)
 {
 	struct kvm_memslots *slots;
@@ -841,10 +846,8 @@ static void unaccount_shadowed(struct kvm *kvm, struct kvm_mmu_page *sp)
 	kvm_mmu_gfn_allow_lpage(slot, gfn);
 }
 
-void unaccount_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp)
+void untrack_possible_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp)
 {
-	sp->nx_huge_page_disallowed = false;
-
 	if (list_empty(&sp->possible_nx_huge_page_link))
 		return;
 
@@ -852,6 +855,13 @@ void unaccount_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp)
 	list_del_init(&sp->possible_nx_huge_page_link);
 }
 
+static void unaccount_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp)
+{
+	sp->nx_huge_page_disallowed = false;
+
+	untrack_possible_nx_huge_page(kvm, sp);
+}
+
 static struct kvm_memory_slot *
 gfn_to_memslot_dirty_bitmap(struct kvm_vcpu *vcpu, gfn_t gfn,
 			    bool no_dirty_log)
diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
index 67879459a25c..22152241bd29 100644
--- a/arch/x86/kvm/mmu/mmu_internal.h
+++ b/arch/x86/kvm/mmu/mmu_internal.h
@@ -328,8 +328,7 @@ void disallowed_hugepage_adjust(struct kvm_page_fault *fault, u64 spte, int cur_
 
 void *mmu_memory_cache_alloc(struct kvm_mmu_memory_cache *mc);
 
-void account_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp,
-			  bool nx_huge_page_possible);
-void unaccount_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp);
+void track_possible_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp);
+void untrack_possible_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp);
 
 #endif /* __KVM_X86_MMU_INTERNAL_H */
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 73eb28ed1f03..059231c82345 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -403,8 +403,11 @@ static void tdp_mmu_unlink_sp(struct kvm *kvm, struct kvm_mmu_page *sp,
 		lockdep_assert_held_write(&kvm->mmu_lock);
 
 	list_del(&sp->link);
-	if (sp->nx_huge_page_disallowed)
-		unaccount_nx_huge_page(kvm, sp);
+
+	if (sp->nx_huge_page_disallowed) {
+		sp->nx_huge_page_disallowed = false;
+		untrack_possible_nx_huge_page(kvm, sp);
+	}
 
 	if (shared)
 		spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
@@ -1118,16 +1121,13 @@ static int tdp_mmu_map_handle_target_level(struct kvm_vcpu *vcpu,
  * @kvm: kvm instance
  * @iter: a tdp_iter instance currently on the SPTE that should be set
  * @sp: The new TDP page table to install.
- * @account_nx: True if this page table is being installed to split a
- *              non-executable huge page.
  * @shared: This operation is running under the MMU lock in read mode.
  *
  * Returns: 0 if the new page table was installed. Non-0 if the page table
  *          could not be installed (e.g. the atomic compare-exchange failed).
  */
 static int tdp_mmu_link_sp(struct kvm *kvm, struct tdp_iter *iter,
-			   struct kvm_mmu_page *sp, bool account_nx,
-			   bool shared)
+			   struct kvm_mmu_page *sp, bool shared)
 {
 	u64 spte = make_nonleaf_spte(sp->spt, !kvm_ad_enabled());
 	int ret = 0;
@@ -1142,8 +1142,6 @@ static int tdp_mmu_link_sp(struct kvm *kvm, struct tdp_iter *iter,
 
 	spin_lock(&kvm->arch.tdp_mmu_pages_lock);
 	list_add(&sp->link, &kvm->arch.tdp_mmu_pages);
-	if (account_nx)
-		account_nx_huge_page(kvm, sp, true);
 	spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
 	tdp_account_mmu_page(kvm, sp);
 
@@ -1157,6 +1155,7 @@ static int tdp_mmu_link_sp(struct kvm *kvm, struct tdp_iter *iter,
 int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 {
 	struct kvm_mmu *mmu = vcpu->arch.mmu;
+	struct kvm *kvm = vcpu->kvm;
 	struct tdp_iter iter;
 	struct kvm_mmu_page *sp;
 	int ret;
@@ -1193,9 +1192,6 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 		}
 
 		if (!is_shadow_present_pte(iter.old_spte)) {
-			bool account_nx = fault->huge_page_disallowed &&
-					  fault->req_level >= iter.level;
-
 			/*
 			 * If SPTE has been frozen by another thread, just
 			 * give up and retry, avoiding unnecessary page table
@@ -1207,10 +1203,19 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 			sp = tdp_mmu_alloc_sp(vcpu);
 			tdp_mmu_init_child_sp(sp, &iter);
 
-			if (tdp_mmu_link_sp(vcpu->kvm, &iter, sp, account_nx, true)) {
+			sp->nx_huge_page_disallowed = fault->huge_page_disallowed;
+
+			if (tdp_mmu_link_sp(kvm, &iter, sp, true)) {
 				tdp_mmu_free_sp(sp);
 				break;
 			}
+
+			if (fault->huge_page_disallowed &&
+			    fault->req_level >= iter.level) {
+				spin_lock(&kvm->arch.tdp_mmu_pages_lock);
+				track_possible_nx_huge_page(kvm, sp);
+				spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
+			}
 		}
 	}
 
@@ -1498,7 +1503,7 @@ static int tdp_mmu_split_huge_page(struct kvm *kvm, struct tdp_iter *iter,
 	 * correctness standpoint since the translation will be the same either
 	 * way.
 	 */
-	ret = tdp_mmu_link_sp(kvm, iter, sp, false, shared);
+	ret = tdp_mmu_link_sp(kvm, iter, sp, shared);
 	if (ret)
 		goto out;
 
-- 
2.38.0.413.g74048e4d9e-goog

Re: [PATCH v6 4/8] KVM: x86/mmu: Set disallowed_nx_huge_page in TDP MMU before setting SPTE

[Yan Zhao]

Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>

On Wed, Oct 19, 2022 at 04:56:14PM +0000, Sean Christopherson wrote:
> Set nx_huge_page_disallowed in TDP MMU shadow pages before making the SP
> visible to other readers, i.e. before setting its SPTE.  This will allow
> KVM to query the flag when determining if a shadow page can be replaced
> by a NX huge page without violating the rules of the mitigation.
> 
> Note, the shadow/legacy MMU holds mmu_lock for write, so it's impossible
> for another CPU to see a shadow page without an up-to-date
> nx_huge_page_disallowed, i.e. only the TDP MMU needs the complicated
> dance.
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Reviewed-by: David Matlack <dmatlack@google.com>
> ---
>  arch/x86/kvm/mmu/mmu.c          | 28 +++++++++++++++++++---------
>  arch/x86/kvm/mmu/mmu_internal.h |  5 ++---
>  arch/x86/kvm/mmu/tdp_mmu.c      | 31 ++++++++++++++++++-------------
>  3 files changed, 39 insertions(+), 25 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 99086a684dd2..57c7c52d137a 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -802,11 +802,8 @@ static void account_shadowed(struct kvm *kvm, struct kvm_mmu_page *sp)
>  		kvm_flush_remote_tlbs_with_address(kvm, gfn, 1);
>  }
>  
> -void account_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp,
> -			  bool nx_huge_page_possible)
> +void track_possible_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp)
>  {
> -	sp->nx_huge_page_disallowed = true;
> -
>  	/*
>  	 * If it's possible to replace the shadow page with an NX huge page,
>  	 * i.e. if the shadow page is the only thing currently preventing KVM
> @@ -815,8 +812,7 @@ void account_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp,
>  	 * on the list if KVM is reusing an existing shadow page, i.e. if KVM
>  	 * links a shadow page at multiple points.
>  	 */
> -	if (!nx_huge_page_possible ||
> -	    !list_empty(&sp->possible_nx_huge_page_link))
> +	if (!list_empty(&sp->possible_nx_huge_page_link))
>  		return;
>  
>  	++kvm->stat.nx_lpage_splits;
> @@ -824,6 +820,15 @@ void account_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp,
>  		      &kvm->arch.possible_nx_huge_pages);
>  }
>  
> +static void account_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp,
> +				 bool nx_huge_page_possible)
> +{
> +	sp->nx_huge_page_disallowed = true;
> +
> +	if (nx_huge_page_possible)
> +		track_possible_nx_huge_page(kvm, sp);
> +}
> +
>  static void unaccount_shadowed(struct kvm *kvm, struct kvm_mmu_page *sp)
>  {
>  	struct kvm_memslots *slots;
> @@ -841,10 +846,8 @@ static void unaccount_shadowed(struct kvm *kvm, struct kvm_mmu_page *sp)
>  	kvm_mmu_gfn_allow_lpage(slot, gfn);
>  }
>  
> -void unaccount_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp)
> +void untrack_possible_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp)
>  {
> -	sp->nx_huge_page_disallowed = false;
> -
>  	if (list_empty(&sp->possible_nx_huge_page_link))
>  		return;
>  
> @@ -852,6 +855,13 @@ void unaccount_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp)
>  	list_del_init(&sp->possible_nx_huge_page_link);
>  }
>  
> +static void unaccount_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp)
> +{
> +	sp->nx_huge_page_disallowed = false;
> +
> +	untrack_possible_nx_huge_page(kvm, sp);
> +}
> +
>  static struct kvm_memory_slot *
>  gfn_to_memslot_dirty_bitmap(struct kvm_vcpu *vcpu, gfn_t gfn,
>  			    bool no_dirty_log)
> diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
> index 67879459a25c..22152241bd29 100644
> --- a/arch/x86/kvm/mmu/mmu_internal.h
> +++ b/arch/x86/kvm/mmu/mmu_internal.h
> @@ -328,8 +328,7 @@ void disallowed_hugepage_adjust(struct kvm_page_fault *fault, u64 spte, int cur_
>  
>  void *mmu_memory_cache_alloc(struct kvm_mmu_memory_cache *mc);
>  
> -void account_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp,
> -			  bool nx_huge_page_possible);
> -void unaccount_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp);
> +void track_possible_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp);
> +void untrack_possible_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp);
>  
>  #endif /* __KVM_X86_MMU_INTERNAL_H */
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 73eb28ed1f03..059231c82345 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -403,8 +403,11 @@ static void tdp_mmu_unlink_sp(struct kvm *kvm, struct kvm_mmu_page *sp,
>  		lockdep_assert_held_write(&kvm->mmu_lock);
>  
>  	list_del(&sp->link);
> -	if (sp->nx_huge_page_disallowed)
> -		unaccount_nx_huge_page(kvm, sp);
> +
> +	if (sp->nx_huge_page_disallowed) {
> +		sp->nx_huge_page_disallowed = false;
> +		untrack_possible_nx_huge_page(kvm, sp);
> +	}
>  
>  	if (shared)
>  		spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
> @@ -1118,16 +1121,13 @@ static int tdp_mmu_map_handle_target_level(struct kvm_vcpu *vcpu,
>   * @kvm: kvm instance
>   * @iter: a tdp_iter instance currently on the SPTE that should be set
>   * @sp: The new TDP page table to install.
> - * @account_nx: True if this page table is being installed to split a
> - *              non-executable huge page.
>   * @shared: This operation is running under the MMU lock in read mode.
>   *
>   * Returns: 0 if the new page table was installed. Non-0 if the page table
>   *          could not be installed (e.g. the atomic compare-exchange failed).
>   */
>  static int tdp_mmu_link_sp(struct kvm *kvm, struct tdp_iter *iter,
> -			   struct kvm_mmu_page *sp, bool account_nx,
> -			   bool shared)
> +			   struct kvm_mmu_page *sp, bool shared)
>  {
>  	u64 spte = make_nonleaf_spte(sp->spt, !kvm_ad_enabled());
>  	int ret = 0;
> @@ -1142,8 +1142,6 @@ static int tdp_mmu_link_sp(struct kvm *kvm, struct tdp_iter *iter,
>  
>  	spin_lock(&kvm->arch.tdp_mmu_pages_lock);
>  	list_add(&sp->link, &kvm->arch.tdp_mmu_pages);
> -	if (account_nx)
> -		account_nx_huge_page(kvm, sp, true);
>  	spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
>  	tdp_account_mmu_page(kvm, sp);
>  
> @@ -1157,6 +1155,7 @@ static int tdp_mmu_link_sp(struct kvm *kvm, struct tdp_iter *iter,
>  int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>  {
>  	struct kvm_mmu *mmu = vcpu->arch.mmu;
> +	struct kvm *kvm = vcpu->kvm;
>  	struct tdp_iter iter;
>  	struct kvm_mmu_page *sp;
>  	int ret;
> @@ -1193,9 +1192,6 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>  		}
>  
>  		if (!is_shadow_present_pte(iter.old_spte)) {
> -			bool account_nx = fault->huge_page_disallowed &&
> -					  fault->req_level >= iter.level;
> -
>  			/*
>  			 * If SPTE has been frozen by another thread, just
>  			 * give up and retry, avoiding unnecessary page table
> @@ -1207,10 +1203,19 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>  			sp = tdp_mmu_alloc_sp(vcpu);
>  			tdp_mmu_init_child_sp(sp, &iter);
>  
> -			if (tdp_mmu_link_sp(vcpu->kvm, &iter, sp, account_nx, true)) {
> +			sp->nx_huge_page_disallowed = fault->huge_page_disallowed;
> +
> +			if (tdp_mmu_link_sp(kvm, &iter, sp, true)) {
>  				tdp_mmu_free_sp(sp);
>  				break;
>  			}
> +
> +			if (fault->huge_page_disallowed &&
> +			    fault->req_level >= iter.level) {
> +				spin_lock(&kvm->arch.tdp_mmu_pages_lock);
> +				track_possible_nx_huge_page(kvm, sp);
> +				spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
> +			}
>  		}
>  	}
>  
> @@ -1498,7 +1503,7 @@ static int tdp_mmu_split_huge_page(struct kvm *kvm, struct tdp_iter *iter,
>  	 * correctness standpoint since the translation will be the same either
>  	 * way.
>  	 */
> -	ret = tdp_mmu_link_sp(kvm, iter, sp, false, shared);
> +	ret = tdp_mmu_link_sp(kvm, iter, sp, shared);
>  	if (ret)
>  		goto out;
>  
> -- 
> 2.38.0.413.g74048e4d9e-goog
> 

[PATCH v6 5/8] KVM: x86/mmu: Track the number of TDP MMU pages, but not the actual pages

[Sean Christopherson]

Track the number of TDP MMU "shadow" pages instead of tracking the pages
themselves. With the NX huge page list manipulation moved out of the common
linking flow, elminating the list-based tracking means the happy path of
adding a shadow page doesn't need to acquire a spinlock and can instead
inc/dec an atomic.

Keep the tracking as the WARN during TDP MMU teardown on leaked shadow
pages is very, very useful for detecting KVM bugs.

Tracking the number of pages will also make it trivial to expose the
counter to userspace as a stat in the future, which may or may not be
desirable.

Note, the TDP MMU needs to use a separate counter (and stat if that ever
comes to be) from the existing n_used_mmu_pages. The TDP MMU doesn't bother
supporting the shrinker nor does it honor KVM_SET_NR_MMU_PAGES (because the
TDP MMU consumes so few pages relative to shadow paging), and including TDP
MMU pages in that counter would break both the shrinker and shadow MMUs,
e.g. if a VM is using nested TDP.

Cc: Yan Zhao <yan.y.zhao@intel.com>
Reviewed-by: Mingwei Zhang <mizhang@google.com>
Reviewed-by: David Matlack <dmatlack@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/include/asm/kvm_host.h | 11 +++--------
 arch/x86/kvm/mmu/tdp_mmu.c      | 20 +++++++++-----------
 2 files changed, 12 insertions(+), 19 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 0333dbb8ec85..bbd2cecd34cb 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1290,6 +1290,9 @@ struct kvm_arch {
 	 */
 	bool tdp_mmu_enabled;
 
+	/* The number of TDP MMU pages across all roots. */
+	atomic64_t tdp_mmu_pages;
+
 	/*
 	 * List of kvm_mmu_page structs being used as roots.
 	 * All kvm_mmu_page structs in the list should have
@@ -1310,18 +1313,10 @@ struct kvm_arch {
 	 */
 	struct list_head tdp_mmu_roots;
 
-	/*
-	 * List of kvm_mmu_page structs not being used as roots.
-	 * All kvm_mmu_page structs in the list should have
-	 * tdp_mmu_page set and a tdp_mmu_root_count of 0.
-	 */
-	struct list_head tdp_mmu_pages;
-
 	/*
 	 * Protects accesses to the following fields when the MMU lock
 	 * is held in read mode:
 	 *  - tdp_mmu_roots (above)
-	 *  - tdp_mmu_pages (above)
 	 *  - the link field of kvm_mmu_page structs used by the TDP MMU
 	 *  - possible_nx_huge_pages;
 	 *  - the possible_nx_huge_page_link field of kvm_mmu_page structs used
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 059231c82345..4e5b3ae824c1 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -29,7 +29,6 @@ int kvm_mmu_init_tdp_mmu(struct kvm *kvm)
 	kvm->arch.tdp_mmu_enabled = true;
 	INIT_LIST_HEAD(&kvm->arch.tdp_mmu_roots);
 	spin_lock_init(&kvm->arch.tdp_mmu_pages_lock);
-	INIT_LIST_HEAD(&kvm->arch.tdp_mmu_pages);
 	kvm->arch.tdp_mmu_zap_wq = wq;
 	return 1;
 }
@@ -54,7 +53,7 @@ void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm)
 	/* Also waits for any queued work items.  */
 	destroy_workqueue(kvm->arch.tdp_mmu_zap_wq);
 
-	WARN_ON(!list_empty(&kvm->arch.tdp_mmu_pages));
+	WARN_ON(atomic64_read(&kvm->arch.tdp_mmu_pages));
 	WARN_ON(!list_empty(&kvm->arch.tdp_mmu_roots));
 
 	/*
@@ -377,11 +376,13 @@ static void handle_changed_spte_dirty_log(struct kvm *kvm, int as_id, gfn_t gfn,
 static void tdp_account_mmu_page(struct kvm *kvm, struct kvm_mmu_page *sp)
 {
 	kvm_account_pgtable_pages((void *)sp->spt, +1);
+	atomic64_inc(&kvm->arch.tdp_mmu_pages);
 }
 
 static void tdp_unaccount_mmu_page(struct kvm *kvm, struct kvm_mmu_page *sp)
 {
 	kvm_account_pgtable_pages((void *)sp->spt, -1);
+	atomic64_dec(&kvm->arch.tdp_mmu_pages);
 }
 
 /**
@@ -397,17 +398,17 @@ static void tdp_mmu_unlink_sp(struct kvm *kvm, struct kvm_mmu_page *sp,
 			      bool shared)
 {
 	tdp_unaccount_mmu_page(kvm, sp);
+
+	if (!sp->nx_huge_page_disallowed)
+		return;
+
 	if (shared)
 		spin_lock(&kvm->arch.tdp_mmu_pages_lock);
 	else
 		lockdep_assert_held_write(&kvm->mmu_lock);
 
-	list_del(&sp->link);
-
-	if (sp->nx_huge_page_disallowed) {
-		sp->nx_huge_page_disallowed = false;
-		untrack_possible_nx_huge_page(kvm, sp);
-	}
+	sp->nx_huge_page_disallowed = false;
+	untrack_possible_nx_huge_page(kvm, sp);
 
 	if (shared)
 		spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
@@ -1140,9 +1141,6 @@ static int tdp_mmu_link_sp(struct kvm *kvm, struct tdp_iter *iter,
 		tdp_mmu_set_spte(kvm, iter, spte);
 	}
 
-	spin_lock(&kvm->arch.tdp_mmu_pages_lock);
-	list_add(&sp->link, &kvm->arch.tdp_mmu_pages);
-	spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
 	tdp_account_mmu_page(kvm, sp);
 
 	return 0;
-- 
2.38.0.413.g74048e4d9e-goog

Re: [PATCH v6 5/8] KVM: x86/mmu: Track the number of TDP MMU pages, but not the actual pages

[Yan Zhao]

Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>

On Wed, Oct 19, 2022 at 04:56:15PM +0000, Sean Christopherson wrote:
> Track the number of TDP MMU "shadow" pages instead of tracking the pages
> themselves. With the NX huge page list manipulation moved out of the common
> linking flow, elminating the list-based tracking means the happy path of
> adding a shadow page doesn't need to acquire a spinlock and can instead
> inc/dec an atomic.
> 
> Keep the tracking as the WARN during TDP MMU teardown on leaked shadow
> pages is very, very useful for detecting KVM bugs.
> 
> Tracking the number of pages will also make it trivial to expose the
> counter to userspace as a stat in the future, which may or may not be
> desirable.
> 
> Note, the TDP MMU needs to use a separate counter (and stat if that ever
> comes to be) from the existing n_used_mmu_pages. The TDP MMU doesn't bother
> supporting the shrinker nor does it honor KVM_SET_NR_MMU_PAGES (because the
> TDP MMU consumes so few pages relative to shadow paging), and including TDP
> MMU pages in that counter would break both the shrinker and shadow MMUs,
> e.g. if a VM is using nested TDP.
> 
> Cc: Yan Zhao <yan.y.zhao@intel.com>
> Reviewed-by: Mingwei Zhang <mizhang@google.com>
> Reviewed-by: David Matlack <dmatlack@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/include/asm/kvm_host.h | 11 +++--------
>  arch/x86/kvm/mmu/tdp_mmu.c      | 20 +++++++++-----------
>  2 files changed, 12 insertions(+), 19 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 0333dbb8ec85..bbd2cecd34cb 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1290,6 +1290,9 @@ struct kvm_arch {
>  	 */
>  	bool tdp_mmu_enabled;
>  
> +	/* The number of TDP MMU pages across all roots. */
> +	atomic64_t tdp_mmu_pages;
> +
>  	/*
>  	 * List of kvm_mmu_page structs being used as roots.
>  	 * All kvm_mmu_page structs in the list should have
> @@ -1310,18 +1313,10 @@ struct kvm_arch {
>  	 */
>  	struct list_head tdp_mmu_roots;
>  
> -	/*
> -	 * List of kvm_mmu_page structs not being used as roots.
> -	 * All kvm_mmu_page structs in the list should have
> -	 * tdp_mmu_page set and a tdp_mmu_root_count of 0.
> -	 */
> -	struct list_head tdp_mmu_pages;
> -
>  	/*
>  	 * Protects accesses to the following fields when the MMU lock
>  	 * is held in read mode:
>  	 *  - tdp_mmu_roots (above)
> -	 *  - tdp_mmu_pages (above)
>  	 *  - the link field of kvm_mmu_page structs used by the TDP MMU
>  	 *  - possible_nx_huge_pages;
>  	 *  - the possible_nx_huge_page_link field of kvm_mmu_page structs used
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 059231c82345..4e5b3ae824c1 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -29,7 +29,6 @@ int kvm_mmu_init_tdp_mmu(struct kvm *kvm)
>  	kvm->arch.tdp_mmu_enabled = true;
>  	INIT_LIST_HEAD(&kvm->arch.tdp_mmu_roots);
>  	spin_lock_init(&kvm->arch.tdp_mmu_pages_lock);
> -	INIT_LIST_HEAD(&kvm->arch.tdp_mmu_pages);
>  	kvm->arch.tdp_mmu_zap_wq = wq;
>  	return 1;
>  }
> @@ -54,7 +53,7 @@ void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm)
>  	/* Also waits for any queued work items.  */
>  	destroy_workqueue(kvm->arch.tdp_mmu_zap_wq);
>  
> -	WARN_ON(!list_empty(&kvm->arch.tdp_mmu_pages));
> +	WARN_ON(atomic64_read(&kvm->arch.tdp_mmu_pages));
>  	WARN_ON(!list_empty(&kvm->arch.tdp_mmu_roots));
>  
>  	/*
> @@ -377,11 +376,13 @@ static void handle_changed_spte_dirty_log(struct kvm *kvm, int as_id, gfn_t gfn,
>  static void tdp_account_mmu_page(struct kvm *kvm, struct kvm_mmu_page *sp)
>  {
>  	kvm_account_pgtable_pages((void *)sp->spt, +1);
> +	atomic64_inc(&kvm->arch.tdp_mmu_pages);
>  }
>  
>  static void tdp_unaccount_mmu_page(struct kvm *kvm, struct kvm_mmu_page *sp)
>  {
>  	kvm_account_pgtable_pages((void *)sp->spt, -1);
> +	atomic64_dec(&kvm->arch.tdp_mmu_pages);
>  }
>  
>  /**
> @@ -397,17 +398,17 @@ static void tdp_mmu_unlink_sp(struct kvm *kvm, struct kvm_mmu_page *sp,
>  			      bool shared)
>  {
>  	tdp_unaccount_mmu_page(kvm, sp);
> +
> +	if (!sp->nx_huge_page_disallowed)
> +		return;
> +
>  	if (shared)
>  		spin_lock(&kvm->arch.tdp_mmu_pages_lock);
>  	else
>  		lockdep_assert_held_write(&kvm->mmu_lock);
>  
> -	list_del(&sp->link);
> -
> -	if (sp->nx_huge_page_disallowed) {
> -		sp->nx_huge_page_disallowed = false;
> -		untrack_possible_nx_huge_page(kvm, sp);
> -	}
> +	sp->nx_huge_page_disallowed = false;
> +	untrack_possible_nx_huge_page(kvm, sp);
>  
>  	if (shared)
>  		spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
> @@ -1140,9 +1141,6 @@ static int tdp_mmu_link_sp(struct kvm *kvm, struct tdp_iter *iter,
>  		tdp_mmu_set_spte(kvm, iter, spte);
>  	}
>  
> -	spin_lock(&kvm->arch.tdp_mmu_pages_lock);
> -	list_add(&sp->link, &kvm->arch.tdp_mmu_pages);
> -	spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
>  	tdp_account_mmu_page(kvm, sp);
>  
>  	return 0;
> -- 
> 2.38.0.413.g74048e4d9e-goog
> 

[PATCH v6 6/8] KVM: x86/mmu: Add helper to convert SPTE value to its shadow page

[Sean Christopherson]

Add a helper to convert a SPTE to its shadow page to deduplicate a
variety of flows and hopefully avoid future bugs, e.g. if KVM attempts to
get the shadow page for a SPTE without dropping high bits.

Opportunistically add a comment in mmu_free_root_page() documenting why
it treats the root HPA as a SPTE.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/mmu/mmu.c          | 17 ++++++++++-------
 arch/x86/kvm/mmu/mmu_internal.h | 12 ------------
 arch/x86/kvm/mmu/spte.h         | 17 +++++++++++++++++
 arch/x86/kvm/mmu/tdp_mmu.h      |  2 ++
 4 files changed, 29 insertions(+), 19 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 57c7c52d137a..f4f1b1591a02 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -1818,7 +1818,7 @@ static int __mmu_unsync_walk(struct kvm_mmu_page *sp,
 			continue;
 		}
 
-		child = to_shadow_page(ent & SPTE_BASE_ADDR_MASK);
+		child = spte_to_child_sp(ent);
 
 		if (child->unsync_children) {
 			if (mmu_pages_add(pvec, child, i))
@@ -2377,7 +2377,7 @@ static void validate_direct_spte(struct kvm_vcpu *vcpu, u64 *sptep,
 		 * so we should update the spte at this point to get
 		 * a new sp with the correct access.
 		 */
-		child = to_shadow_page(*sptep & SPTE_BASE_ADDR_MASK);
+		child = spte_to_child_sp(*sptep);
 		if (child->role.access == direct_access)
 			return;
 
@@ -2398,7 +2398,7 @@ static int mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp,
 		if (is_last_spte(pte, sp->role.level)) {
 			drop_spte(kvm, spte);
 		} else {
-			child = to_shadow_page(pte & SPTE_BASE_ADDR_MASK);
+			child = spte_to_child_sp(pte);
 			drop_parent_pte(child, spte);
 
 			/*
@@ -2837,7 +2837,7 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, struct kvm_memory_slot *slot,
 			struct kvm_mmu_page *child;
 			u64 pte = *sptep;
 
-			child = to_shadow_page(pte & SPTE_BASE_ADDR_MASK);
+			child = spte_to_child_sp(pte);
 			drop_parent_pte(child, sptep);
 			flush = true;
 		} else if (pfn != spte_to_pfn(*sptep)) {
@@ -3449,7 +3449,11 @@ static void mmu_free_root_page(struct kvm *kvm, hpa_t *root_hpa,
 	if (!VALID_PAGE(*root_hpa))
 		return;
 
-	sp = to_shadow_page(*root_hpa & SPTE_BASE_ADDR_MASK);
+	/*
+	 * The "root" may be a special root, e.g. a PAE entry, treat it as a
+	 * SPTE to ensure any non-PA bits are dropped.
+	 */
+	sp = spte_to_child_sp(*root_hpa);
 	if (WARN_ON(!sp))
 		return;
 
@@ -3934,8 +3938,7 @@ void kvm_mmu_sync_roots(struct kvm_vcpu *vcpu)
 		hpa_t root = vcpu->arch.mmu->pae_root[i];
 
 		if (IS_VALID_PAE_ROOT(root)) {
-			root &= SPTE_BASE_ADDR_MASK;
-			sp = to_shadow_page(root);
+			sp = spte_to_child_sp(root);
 			mmu_sync_children(vcpu, sp, true);
 		}
 	}
diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
index 22152241bd29..dbaf6755c5a7 100644
--- a/arch/x86/kvm/mmu/mmu_internal.h
+++ b/arch/x86/kvm/mmu/mmu_internal.h
@@ -133,18 +133,6 @@ struct kvm_mmu_page {
 
 extern struct kmem_cache *mmu_page_header_cache;
 
-static inline struct kvm_mmu_page *to_shadow_page(hpa_t shadow_page)
-{
-	struct page *page = pfn_to_page(shadow_page >> PAGE_SHIFT);
-
-	return (struct kvm_mmu_page *)page_private(page);
-}
-
-static inline struct kvm_mmu_page *sptep_to_sp(u64 *sptep)
-{
-	return to_shadow_page(__pa(sptep));
-}
-
 static inline int kvm_mmu_role_as_id(union kvm_mmu_page_role role)
 {
 	return role.smm ? 1 : 0;
diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
index 7670c13ce251..7e5343339b90 100644
--- a/arch/x86/kvm/mmu/spte.h
+++ b/arch/x86/kvm/mmu/spte.h
@@ -219,6 +219,23 @@ static inline int spte_index(u64 *sptep)
  */
 extern u64 __read_mostly shadow_nonpresent_or_rsvd_lower_gfn_mask;
 
+static inline struct kvm_mmu_page *to_shadow_page(hpa_t shadow_page)
+{
+	struct page *page = pfn_to_page((shadow_page) >> PAGE_SHIFT);
+
+	return (struct kvm_mmu_page *)page_private(page);
+}
+
+static inline struct kvm_mmu_page *spte_to_child_sp(u64 spte)
+{
+	return to_shadow_page(spte & SPTE_BASE_ADDR_MASK);
+}
+
+static inline struct kvm_mmu_page *sptep_to_sp(u64 *sptep)
+{
+	return to_shadow_page(__pa(sptep));
+}
+
 static inline bool is_mmio_spte(u64 spte)
 {
 	return (spte & shadow_mmio_mask) == shadow_mmio_value &&
diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
index c163f7cc23ca..d3714200b932 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.h
+++ b/arch/x86/kvm/mmu/tdp_mmu.h
@@ -5,6 +5,8 @@
 
 #include <linux/kvm_host.h>
 
+#include "spte.h"
+
 hpa_t kvm_tdp_mmu_get_vcpu_root_hpa(struct kvm_vcpu *vcpu);
 
 __must_check static inline bool kvm_tdp_mmu_get_root(struct kvm_mmu_page *root)
-- 
2.38.0.413.g74048e4d9e-goog

[PATCH v6 7/8] KVM: x86/mmu: explicitly check nx_hugepage in disallowed_hugepage_adjust()

[Sean Christopherson]

From: Mingwei Zhang <mizhang@google.com>

Explicitly check if a NX huge page is disallowed when determining if a
page fault needs to be forced to use a smaller sized page.  KVM currently
assumes that the NX huge page mitigation is the only scenario where KVM
will force a shadow page instead of a huge page, and so unnecessarily
keeps an existing shadow page instead of replacing it with a huge page.

Any scenario that causes KVM to zap leaf SPTEs may result in having a SP
that can be made huge without violating the NX huge page mitigation.
E.g. prior to commit 5ba7c4c6d1c7 ("KVM: x86/MMU: Zap non-leaf SPTEs when
disabling dirty logging"), KVM would keep shadow pages after disabling
dirty logging due to a live migration being canceled, resulting in
degraded performance due to running with 4kb pages instead of huge pages.

Although the dirty logging case is "fixed", that fix is coincidental,
i.e. is an implementation detail, and there are other scenarios where KVM
will zap leaf SPTEs.  E.g. zapping leaf SPTEs in response to a host page
migration (mmu_notifier invalidation) to create a huge page would yield a
similar result; KVM would see the shadow-present non-leaf SPTE and assume
a huge page is disallowed.

Fixes: b8e8c8303ff2 ("kvm: mmu: ITLB_MULTIHIT mitigation")
Reviewed-by: Ben Gardon <bgardon@google.com>
Reviewed-by: David Matlack <dmatlack@google.com>
Signed-off-by: Mingwei Zhang <mizhang@google.com>
[sean: use spte_to_child_sp(), massage changelog, fold into if-statement]
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/mmu/mmu.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index f4f1b1591a02..14674c9e10f7 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3111,7 +3111,8 @@ void disallowed_hugepage_adjust(struct kvm_page_fault *fault, u64 spte, int cur_
 	if (cur_level > PG_LEVEL_4K &&
 	    cur_level == fault->goal_level &&
 	    is_shadow_present_pte(spte) &&
-	    !is_large_pte(spte)) {
+	    !is_large_pte(spte) &&
+	    spte_to_child_sp(spte)->nx_huge_page_disallowed) {
 		/*
 		 * A small SPTE exists for this pfn, but FNAME(fetch)
 		 * and __direct_map would like to create a large PTE
-- 
2.38.0.413.g74048e4d9e-goog

Re: [PATCH v6 7/8] KVM: x86/mmu: explicitly check nx_hugepage in disallowed_hugepage_adjust()

[Yan Zhao]

Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>

On Wed, Oct 19, 2022 at 04:56:17PM +0000, Sean Christopherson wrote:
> From: Mingwei Zhang <mizhang@google.com>
> 
> Explicitly check if a NX huge page is disallowed when determining if a
> page fault needs to be forced to use a smaller sized page.  KVM currently
> assumes that the NX huge page mitigation is the only scenario where KVM
> will force a shadow page instead of a huge page, and so unnecessarily
> keeps an existing shadow page instead of replacing it with a huge page.
> 
> Any scenario that causes KVM to zap leaf SPTEs may result in having a SP
> that can be made huge without violating the NX huge page mitigation.
> E.g. prior to commit 5ba7c4c6d1c7 ("KVM: x86/MMU: Zap non-leaf SPTEs when
> disabling dirty logging"), KVM would keep shadow pages after disabling
> dirty logging due to a live migration being canceled, resulting in
> degraded performance due to running with 4kb pages instead of huge pages.
> 
> Although the dirty logging case is "fixed", that fix is coincidental,
> i.e. is an implementation detail, and there are other scenarios where KVM
> will zap leaf SPTEs.  E.g. zapping leaf SPTEs in response to a host page
> migration (mmu_notifier invalidation) to create a huge page would yield a
> similar result; KVM would see the shadow-present non-leaf SPTE and assume
> a huge page is disallowed.
> 
> Fixes: b8e8c8303ff2 ("kvm: mmu: ITLB_MULTIHIT mitigation")
> Reviewed-by: Ben Gardon <bgardon@google.com>
> Reviewed-by: David Matlack <dmatlack@google.com>
> Signed-off-by: Mingwei Zhang <mizhang@google.com>
> [sean: use spte_to_child_sp(), massage changelog, fold into if-statement]
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/kvm/mmu/mmu.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index f4f1b1591a02..14674c9e10f7 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3111,7 +3111,8 @@ void disallowed_hugepage_adjust(struct kvm_page_fault *fault, u64 spte, int cur_
>  	if (cur_level > PG_LEVEL_4K &&
>  	    cur_level == fault->goal_level &&
>  	    is_shadow_present_pte(spte) &&
> -	    !is_large_pte(spte)) {
> +	    !is_large_pte(spte) &&
> +	    spte_to_child_sp(spte)->nx_huge_page_disallowed) {
>  		/*
>  		 * A small SPTE exists for this pfn, but FNAME(fetch)
>  		 * and __direct_map would like to create a large PTE
> -- 
> 2.38.0.413.g74048e4d9e-goog
> 

[PATCH v6 8/8] KVM: x86/mmu: WARN if TDP MMU SP disallows hugepage after being zapped

[Sean Christopherson]

Extend the accounting sanity check in kvm_recover_nx_huge_pages() to the
TDP MMU, i.e. verify that zapping a shadow page unaccounts the disallowed
NX huge page regardless of the MMU type.  Recovery runs while holding
mmu_lock for write and so it should be impossible to get false positives
on the WARN.

Suggested-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/mmu/mmu.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 14674c9e10f7..dfd1656232ad 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -6864,12 +6864,11 @@ static void kvm_recover_nx_huge_pages(struct kvm *kvm)
 				      struct kvm_mmu_page,
 				      possible_nx_huge_page_link);
 		WARN_ON_ONCE(!sp->nx_huge_page_disallowed);
-		if (is_tdp_mmu_page(sp)) {
+		if (is_tdp_mmu_page(sp))
 			flush |= kvm_tdp_mmu_zap_sp(kvm, sp);
-		} else {
+		else
 			kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list);
-			WARN_ON_ONCE(sp->nx_huge_page_disallowed);
-		}
+		WARN_ON_ONCE(sp->nx_huge_page_disallowed);
 
 		if (need_resched() || rwlock_needbreak(&kvm->mmu_lock)) {
 			kvm_mmu_remote_flush_or_zap(kvm, &invalid_list, flush);
-- 
2.38.0.413.g74048e4d9e-goog

Re: [PATCH v6 8/8] KVM: x86/mmu: WARN if TDP MMU SP disallows hugepage after being zapped

[Yan Zhao]

Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>

On Wed, Oct 19, 2022 at 04:56:18PM +0000, Sean Christopherson wrote:
> Extend the accounting sanity check in kvm_recover_nx_huge_pages() to the
> TDP MMU, i.e. verify that zapping a shadow page unaccounts the disallowed
> NX huge page regardless of the MMU type.  Recovery runs while holding
> mmu_lock for write and so it should be impossible to get false positives
> on the WARN.
> 
> Suggested-by: Yan Zhao <yan.y.zhao@intel.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/kvm/mmu/mmu.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 14674c9e10f7..dfd1656232ad 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -6864,12 +6864,11 @@ static void kvm_recover_nx_huge_pages(struct kvm *kvm)
>  				      struct kvm_mmu_page,
>  				      possible_nx_huge_page_link);
>  		WARN_ON_ONCE(!sp->nx_huge_page_disallowed);
> -		if (is_tdp_mmu_page(sp)) {
> +		if (is_tdp_mmu_page(sp))
>  			flush |= kvm_tdp_mmu_zap_sp(kvm, sp);
> -		} else {
> +		else
>  			kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list);
> -			WARN_ON_ONCE(sp->nx_huge_page_disallowed);
> -		}
> +		WARN_ON_ONCE(sp->nx_huge_page_disallowed);
>  
>  		if (need_resched() || rwlock_needbreak(&kvm->mmu_lock)) {
>  			kvm_mmu_remote_flush_or_zap(kvm, &invalid_list, flush);
> -- 
> 2.38.0.413.g74048e4d9e-goog
> 

Re: [PATCH v6 0/8] KVM: x86: Apply NX mitigation more precisely

[Paolo Bonzini]

Queued, thanks.

Paolo