[PATCH 24/28] KVM: x86/mmu: Add dedicated helper to zap TDP MMU root shadow page

Sean Christopherson
2021-11-20T04:52:07Z

Convert tdp_mmu_zap_root() into its own dedicated flow instead of simply
redirecting into zap_gfn_range(). In addition to hardening zapping of
roots, this will allow future simplification of zap_gfn_range() by having
it zap only leaf SPTEs, and by removing its tricky "zap all" heuristic.
By having all paths that truly need to free _all_ SPs flow through the
dedicated root zapper, the generic zapper can be freed of those concerns.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/mmu/tdp_mmu.c | 91 +++++++++++++++++++++++++++-----------
1 file changed, 66 insertions(+), 25 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 99ea19e763da..0e5a0d40e54a 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -53,10 +53,6 @@ void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm)
rcu_barrier();
}

-static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
- gfn_t start, gfn_t end, bool can_yield, bool flush,
- bool shared);
-
static void tdp_mmu_free_sp(struct kvm_mmu_page *sp)
{
free_page((unsigned long)sp->spt);
@@ -79,11 +75,8 @@ static void tdp_mmu_free_sp_rcu_callback(struct rcu_head *head)
tdp_mmu_free_sp(sp);
}

-static bool tdp_mmu_zap_root(struct kvm *kvm, struct kvm_mmu_page *root,
- bool shared)
-{
- return zap_gfn_range(kvm, root, 0, -1ull, true, false, shared);
-}
+static void tdp_mmu_zap_root(struct kvm *kvm, struct kvm_mmu_page *root,
+ bool shared, bool root_is_unreachable);

/*
* Note, putting a root might sleep, i.e. the caller must have IRQs enabled and
@@ -120,13 +113,8 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
* invalidates any paging-structure-cache entries, i.e. TLB entries for
* intermediate paging structures, that may be zapped, as such entries
* are associated with the ASID on both VMX and SVM.
- *
- * WARN if a flush is reported for an invalid root, as its child SPTEs
- * should have been zapped by kvm_tdp_mmu_zap_invalidated_roots(), and
- * inserting new SPTEs under an invalid root is a KVM bug.
*/
- if (tdp_mmu_zap_root(kvm, root, shared))
- WARN_ON_ONCE(root->role.invalid);
+ tdp_mmu_zap_root(kvm, root, shared, true);

call_rcu(&root->rcu_head, tdp_mmu_free_sp_rcu_callback);
}
@@ -766,6 +754,65 @@ static inline bool tdp_mmu_iter_cond_resched(struct kvm *kvm,
return false;
}

+static inline gfn_t tdp_mmu_max_gfn_host(void)
+{
+ /*
+ * Bound TDP MMU walks at host.MAXPHYADDR, guest accesses beyond that
+ * will hit a #PF(RSVD) and never hit an EPT Violation/Misconfig / #NPF,
+ * and so KVM will never install a SPTE for such addresses.
+ */
+ return 1ULL << (shadow_phys_bits - PAGE_SHIFT);
+}
+
+static void tdp_mmu_zap_root(struct kvm *kvm, struct kvm_mmu_page *root,
+ bool shared, bool root_is_unreachable)
+{
+ struct tdp_iter iter;
+
+ gfn_t end = tdp_mmu_max_gfn_host();
+ gfn_t start = 0;
+
+ kvm_lockdep_assert_mmu_lock_held(kvm, shared);
+
+ rcu_read_lock();
+
+ /*
+ * No need to try to step down in the iterator when zapping an entire
+ * root, zapping an upper-level SPTE will recurse on its children.
+ */
+ for_each_tdp_pte_min_level(iter, root->spt, root->role.level,
+ root->role.level, start, end) {
+retry:
+ if (tdp_mmu_iter_cond_resched(kvm, &iter, false, shared))
+ continue;
+
+ if (!is_shadow_present_pte(iter.old_spte))
+ continue;
+
+ if (!shared) {
+ tdp_mmu_set_spte(kvm, &iter, 0);
+ } else if (!tdp_mmu_set_spte_atomic(kvm, &iter, 0)) {
+ /*
+ * cmpxchg() shouldn't fail if the root is unreachable.
+ * to be unreachable. Re-read the SPTE and retry so as
+ * not to leak the page and its children.
+ */
+ WARN_ONCE(root_is_unreachable,
+ "Contended TDP MMU SPTE in unreachable root.");
+ iter.old_spte = kvm_tdp_mmu_read_spte(iter.sptep);
+ goto retry;
+ }
+ /*
+ * WARN if the root is invalid and is unreachable, all SPTEs
+ * should've been zapped by kvm_tdp_mmu_zap_invalidated_roots(),
+ * and inserting new SPTEs under an invalid root is a KVM bug.
+ */
+ WARN_ON_ONCE(root_is_unreachable && root->role.invalid);
+ }
+
+ rcu_read_unlock();
+}
+
bool kvm_tdp_mmu_zap_sp(struct kvm *kvm, struct kvm_mmu_page *sp)
{
u64 old_spte;
@@ -807,8 +854,7 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
gfn_t start, gfn_t end, bool can_yield, bool flush,
bool shared)
{
- gfn_t max_gfn_host = 1ULL << (shadow_phys_bits - PAGE_SHIFT);
- bool zap_all = (start == 0 && end >= max_gfn_host);
+ bool zap_all = (start == 0 && end >= tdp_mmu_max_gfn_host());
struct tdp_iter iter;

/*
@@ -817,12 +863,7 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
*/
int min_level = zap_all ? root->role.level : PG_LEVEL_4K;

- /*
- * Bound the walk at host.MAXPHYADDR, guest accesses beyond that will
- * hit a #PF(RSVD) and never get to an EPT Violation/Misconfig / #NPF,
- * and so KVM will never install a SPTE for such addresses.
- */
- end = min(end, max_gfn_host);
+ end = min(end, tdp_mmu_max_gfn_host());

kvm_lockdep_assert_mmu_lock_held(kvm, shared);

@@ -898,7 +939,7 @@ void kvm_tdp_mmu_zap_all(struct kvm *kvm)
*/
for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
for_each_tdp_mmu_root_yield_safe(kvm, root, i, false)
- (void)tdp_mmu_zap_root(kvm, root, false);
+ tdp_mmu_zap_root(kvm, root, false, true);
}
}

@@ -934,7 +975,7 @@ void kvm_tdp_mmu_zap_invalidated_roots(struct kvm *kvm,
* will still flush on yield, but that's a minor performance
* blip and not a functional issue.
*/
- (void)tdp_mmu_zap_root(kvm, root, true);
+ tdp_mmu_zap_root(kvm, root, true, false);
kvm_tdp_mmu_put_root(kvm, root, true);
}
}
--
2.34.0.rc2.393.gf8c9666880-goog