[PATCH v2 0/2] KVM: x86/mmu: Zap orphaned kids for nested TDP MMU

[PATCH v2 0/2] KVM: x86/mmu: Zap orphaned kids for nested TDP MMU

[Sean Christopherson]

As promised, albeit a few days late.

Ben, I kept your performance numbers even though it this version has
non-trivial differences relative to what you tested.  I assume we'll need
a v3 anyways if this doesn't provide the advertised performance benefits.

Ben Gardon (1):
  KVM: x86/MMU: Recursively zap nested TDP SPs when zapping last/only
    parent

Sean Christopherson (1):
  KVM: x86/mmu: Move flush logic from mmu_page_zap_pte() to
    FNAME(invlpg)

 arch/x86/kvm/mmu/mmu.c         | 38 ++++++++++++++++++++++------------
 arch/x86/kvm/mmu/paging_tmpl.h |  7 +++++--
 2 files changed, 30 insertions(+), 15 deletions(-)

-- 
2.28.0

[PATCH v2 1/2] KVM: x86/mmu: Move flush logic from mmu_page_zap_pte() to FNAME(invlpg)

[Sean Christopherson]

Move the logic that controls whether or not FNAME(invlpg) needs to flush
fully into FNAME(invlpg) so that mmu_page_zap_pte() doesn't return a
value.  This allows a future patch to redefine the return semantics for
mmu_page_zap_pte() so that it can recursively zap orphaned child shadow
pages for nested TDP MMUs.

No functional change intended.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---
 arch/x86/kvm/mmu/mmu.c         | 10 +++-------
 arch/x86/kvm/mmu/paging_tmpl.h |  7 +++++--
 2 files changed, 8 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 4e03841f053de..38180befce321 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2614,7 +2614,7 @@ static void validate_direct_spte(struct kvm_vcpu *vcpu, u64 *sptep,
 	}
 }
 
-static bool mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp,
+static void mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp,
 			     u64 *spte)
 {
 	u64 pte;
@@ -2630,13 +2630,9 @@ static bool mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp,
 			child = to_shadow_page(pte & PT64_BASE_ADDR_MASK);
 			drop_parent_pte(child, spte);
 		}
-		return true;
-	}
-
-	if (is_mmio_spte(pte))
+	} else if (is_mmio_spte(pte)) {
 		mmu_spte_clear_no_track(spte);
-
-	return false;
+	}
 }
 
 static void kvm_mmu_page_unlink_children(struct kvm *kvm,
diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index 4dd6b1e5b8cf7..3bb624a3dda92 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -895,6 +895,7 @@ static void FNAME(invlpg)(struct kvm_vcpu *vcpu, gva_t gva, hpa_t root_hpa)
 {
 	struct kvm_shadow_walk_iterator iterator;
 	struct kvm_mmu_page *sp;
+	u64 old_spte;
 	int level;
 	u64 *sptep;
 
@@ -917,7 +918,8 @@ static void FNAME(invlpg)(struct kvm_vcpu *vcpu, gva_t gva, hpa_t root_hpa)
 		sptep = iterator.sptep;
 
 		sp = sptep_to_sp(sptep);
-		if (is_last_spte(*sptep, level)) {
+		old_spte = *sptep;
+		if (is_last_spte(old_spte, level)) {
 			pt_element_t gpte;
 			gpa_t pte_gpa;
 
@@ -927,7 +929,8 @@ static void FNAME(invlpg)(struct kvm_vcpu *vcpu, gva_t gva, hpa_t root_hpa)
 			pte_gpa = FNAME(get_level1_sp_gpa)(sp);
 			pte_gpa += (sptep - sp->spt) * sizeof(pt_element_t);
 
-			if (mmu_page_zap_pte(vcpu->kvm, sp, sptep))
+			mmu_page_zap_pte(vcpu->kvm, sp, sptep);
+			if (is_shadow_present_pte(old_spte))
 				kvm_flush_remote_tlbs_with_address(vcpu->kvm,
 					sp->gfn, KVM_PAGES_PER_HPAGE(sp->role.level));
 
-- 
2.28.0

[PATCH v2 2/2] KVM: x86/MMU: Recursively zap nested TDP SPs when zapping last/only parent

[Sean Christopherson]

From: Ben Gardon <bgardon@google.com>

Recursively zap all to-be-orphaned children, unsynced or otherwise, when
zapping a shadow page for a nested TDP MMU.  KVM currently only zaps the
unsynced child pages, but not the synced ones.  This can create problems
over time when running many nested guests because it leaves unlinked
pages which will not be freed until the page quota is hit. With the
default page quota of 20 shadow pages per 1000 guest pages, this looks
like a memory leak and can degrade MMU performance.

In a recent benchmark, substantial performance degradation was observed:
An L1 guest was booted with 64G memory.
2G nested Windows guests were booted, 10 at a time for 20
iterations. (200 total boots)
Windows was used in this benchmark because they touch all of their
memory on startup.
By the end of the benchmark, the nested guests were taking ~10% longer
to boot. With this patch there is no degradation in boot time.
Without this patch the benchmark ends with hundreds of thousands of
stale EPT02 pages cluttering up rmaps and the page hash map. As a
result, VM shutdown is also much slower: deleting memslot 0 was
observed to take over a minute. With this patch it takes just a
few miliseconds.

Cc: Peter Shier <pshier@google.com>
Signed-off-by: Ben Gardon <bgardon@google.com>
Co-developed-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---
 arch/x86/kvm/mmu/mmu.c         | 30 +++++++++++++++++++++++-------
 arch/x86/kvm/mmu/paging_tmpl.h |  2 +-
 2 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 38180befce321..87f1e73a8d365 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2614,8 +2614,9 @@ static void validate_direct_spte(struct kvm_vcpu *vcpu, u64 *sptep,
 	}
 }
 
-static void mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp,
-			     u64 *spte)
+/* Returns the number of zapped non-leaf child shadow pages. */
+static int mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp,
+			    u64 *spte, struct list_head *invalid_list)
 {
 	u64 pte;
 	struct kvm_mmu_page *child;
@@ -2629,19 +2630,34 @@ static void mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp,
 		} else {
 			child = to_shadow_page(pte & PT64_BASE_ADDR_MASK);
 			drop_parent_pte(child, spte);
+
+			/*
+			 * Recursively zap nested TDP SPs, parentless SPs are
+			 * unlikely to be used again in the near future.  This
+			 * avoids retaining a large number of stale nested SPs.
+			 */
+			if (tdp_enabled && invalid_list &&
+			    child->role.guest_mode && !child->parent_ptes.val)
+				return kvm_mmu_prepare_zap_page(kvm, child,
+								invalid_list);
 		}
 	} else if (is_mmio_spte(pte)) {
 		mmu_spte_clear_no_track(spte);
 	}
+	return 0;
 }
 
-static void kvm_mmu_page_unlink_children(struct kvm *kvm,
-					 struct kvm_mmu_page *sp)
+static int kvm_mmu_page_unlink_children(struct kvm *kvm,
+					struct kvm_mmu_page *sp,
+					struct list_head *invalid_list)
 {
+	int zapped = 0;
 	unsigned i;
 
 	for (i = 0; i < PT64_ENT_PER_PAGE; ++i)
-		mmu_page_zap_pte(kvm, sp, sp->spt + i);
+		zapped += mmu_page_zap_pte(kvm, sp, sp->spt + i, invalid_list);
+
+	return zapped;
 }
 
 static void kvm_mmu_unlink_parents(struct kvm *kvm, struct kvm_mmu_page *sp)
@@ -2687,7 +2703,7 @@ static bool __kvm_mmu_prepare_zap_page(struct kvm *kvm,
 	trace_kvm_mmu_prepare_zap_page(sp);
 	++kvm->stat.mmu_shadow_zapped;
 	*nr_zapped = mmu_zap_unsync_children(kvm, sp, invalid_list);
-	kvm_mmu_page_unlink_children(kvm, sp);
+	*nr_zapped += kvm_mmu_page_unlink_children(kvm, sp, invalid_list);
 	kvm_mmu_unlink_parents(kvm, sp);
 
 	/* Zapping children means active_mmu_pages has become unstable. */
@@ -5395,7 +5411,7 @@ static void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
 			u32 base_role = vcpu->arch.mmu->mmu_role.base.word;
 
 			entry = *spte;
-			mmu_page_zap_pte(vcpu->kvm, sp, spte);
+			mmu_page_zap_pte(vcpu->kvm, sp, spte, NULL);
 			if (gentry &&
 			    !((sp->role.word ^ base_role) & ~role_ign.word) &&
 			    rmap_can_add(vcpu))
diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index 3bb624a3dda92..e1066226b8f0c 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -929,7 +929,7 @@ static void FNAME(invlpg)(struct kvm_vcpu *vcpu, gva_t gva, hpa_t root_hpa)
 			pte_gpa = FNAME(get_level1_sp_gpa)(sp);
 			pte_gpa += (sptep - sp->spt) * sizeof(pt_element_t);
 
-			mmu_page_zap_pte(vcpu->kvm, sp, sptep);
+			mmu_page_zap_pte(vcpu->kvm, sp, sptep, NULL);
 			if (is_shadow_present_pte(old_spte))
 				kvm_flush_remote_tlbs_with_address(vcpu->kvm,
 					sp->gfn, KVM_PAGES_PER_HPAGE(sp->role.level));
-- 
2.28.0

Re: [PATCH v2 0/2] KVM: x86/mmu: Zap orphaned kids for nested TDP MMU

[Ben Gardon]

On Wed, Aug 12, 2020 at 12:28 PM Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> As promised, albeit a few days late.
>
> Ben, I kept your performance numbers even though it this version has
> non-trivial differences relative to what you tested.  I assume we'll need
> a v3 anyways if this doesn't provide the advertised performance benefits.
>
> Ben Gardon (1):
>   KVM: x86/MMU: Recursively zap nested TDP SPs when zapping last/only
>     parent
>
> Sean Christopherson (1):
>   KVM: x86/mmu: Move flush logic from mmu_page_zap_pte() to
>     FNAME(invlpg)
>
>  arch/x86/kvm/mmu/mmu.c         | 38 ++++++++++++++++++++++------------
>  arch/x86/kvm/mmu/paging_tmpl.h |  7 +++++--
>  2 files changed, 30 insertions(+), 15 deletions(-)
>

Thanks for sending this revised series Sean. This all looks good to me.
I think the main performance difference between this series and the
original patch I sent is only zapping nested TDP shadow pages, but I
expect it to behave more or less the same since the number of direct
TDP pages is pretty bounded.

>
> --
> 2.28.0
>