[PATCH v3 22/28] KVM: x86/mmu: Zap defunct roots via asynchronous worker

Sean Christopherson
2022-02-26T00:18:38Z

Zap defunct roots, a.k.a. roots that have been invalidated after their
last reference was initially dropped, asynchronously via the system work
queue instead of forcing the work upon the unfortunate task that happened
to drop the last reference.

If a vCPU task drops the last reference, the vCPU is effectively blocked
by the host for the entire duration of the zap. If the root being zapped
happens be fully populated with 4kb leaf SPTEs, e.g. due to dirty logging
being active, the zap can take several hundred seconds. Unsurprisingly,
most guests are unhappy if a vCPU disappears for hundreds of seconds.

E.g. running a synthetic selftest that triggers a vCPU root zap with
~64tb of guest memory and 4kb SPTEs blocks the vCPU for 900+ seconds.
Offloading the zap to a worker drops the block time to <100ms.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/mmu/mmu_internal.h | 8 +++-
arch/x86/kvm/mmu/tdp_mmu.c | 65 ++++++++++++++++++++++++++++-----
2 files changed, 63 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
index be063b6c91b7..1bff453f7cbe 100644
--- a/arch/x86/kvm/mmu/mmu_internal.h
+++ b/arch/x86/kvm/mmu/mmu_internal.h
@@ -65,7 +65,13 @@ struct kvm_mmu_page {
struct kvm_rmap_head parent_ptes; /* rmap pointers to parent sptes */
tdp_ptep_t ptep;
};
- DECLARE_BITMAP(unsync_child_bitmap, 512);
+ union {
+ DECLARE_BITMAP(unsync_child_bitmap, 512);
+ struct {
+ struct work_struct tdp_mmu_async_work;
+ void *tdp_mmu_async_data;
+ };
+ };

struct list_head lpage_disallowed_link;
#ifdef CONFIG_X86_32
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index ec28a88c6376..4151e61245a7 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -81,6 +81,38 @@ static void tdp_mmu_free_sp_rcu_callback(struct rcu_head *head)
static void tdp_mmu_zap_root(struct kvm *kvm, struct kvm_mmu_page *root,
bool shared);

+static void tdp_mmu_zap_root_async(struct work_struct *work)
+{
+ struct kvm_mmu_page *root = container_of(work, struct kvm_mmu_page,
+ tdp_mmu_async_work);
+ struct kvm *kvm = root->tdp_mmu_async_data;
+
+ read_lock(&kvm->mmu_lock);
+
+ /*
+ * A TLB flush is not necessary as KVM performs a local TLB flush when
+ * allocating a new root (see kvm_mmu_load()), and when migrating vCPU
+ * to a different pCPU. Note, the local TLB flush on reuse also
+ * 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.
+ */
+ tdp_mmu_zap_root(kvm, root, true);
+
+ /*
+ * Drop the refcount using kvm_tdp_mmu_put_root() to test its logic for
+ * avoiding an infinite loop. By design, the root is reachable while
+ * it's being asynchronously zapped, thus a different task can put its
+ * last reference, i.e. flowing through kvm_tdp_mmu_put_root() for an
+ * asynchronously zapped root is unavoidable.
+ */
+ kvm_tdp_mmu_put_root(kvm, root, true);
+
+ read_unlock(&kvm->mmu_lock);
+
+ kvm_put_kvm(kvm);
+}
+
void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
bool shared)
{
@@ -142,15 +174,26 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
refcount_set(&root->tdp_mmu_root_count, 1);

/*
- * Zap the root, then put the refcount "acquired" above. Recursively
- * call kvm_tdp_mmu_put_root() to test the above logic for avoiding an
- * infinite loop by freeing invalid roots. By design, the root is
- * reachable while it's being zapped, thus a different task can put its
- * last reference, i.e. flowing through kvm_tdp_mmu_put_root() for a
- * defunct root is unavoidable.
+ * Attempt to acquire a reference to KVM itself. If KVM is alive, then
+ * zap the root asynchronously in a worker, otherwise it must be zapped
+ * directly here. Wait to do this check until after the refcount is
+ * reset so that tdp_mmu_zap_root() can safely yield.
+ *
+ * In both flows, zap the root, then put the refcount "acquired" above.
+ * When putting the reference, use kvm_tdp_mmu_put_root() to test the
+ * above logic for avoiding an infinite loop by freeing invalid roots.
+ * By design, the root is reachable while it's being zapped, thus a
+ * different task can put its last reference, i.e. flowing through
+ * kvm_tdp_mmu_put_root() for a defunct root is unavoidable.
*/
- tdp_mmu_zap_root(kvm, root, shared);
- kvm_tdp_mmu_put_root(kvm, root, shared);
+ if (kvm_get_kvm_safe(kvm)) {
+ root->tdp_mmu_async_data = kvm;
+ INIT_WORK(&root->tdp_mmu_async_work, tdp_mmu_zap_root_async);
+ schedule_work(&root->tdp_mmu_async_work);
+ } else {
+ tdp_mmu_zap_root(kvm, root, shared);
+ kvm_tdp_mmu_put_root(kvm, root, shared);
+ }
}

enum tdp_mmu_roots_iter_type {
@@ -954,7 +997,11 @@ void kvm_tdp_mmu_zap_all(struct kvm *kvm)

/*
* Zap all roots, including invalid roots, as all SPTEs must be dropped
- * before returning to the caller.
+ * before returning to the caller. Zap directly even if the root is
+ * also being zapped by a worker. Walking zapped top-level SPTEs isn't
+ * all that expensive and mmu_lock is already held, which means the
+ * worker has yielded, i.e. flushing the work instead of zapping here
+ * isn't guaranteed to be any faster.
*
* A TLB flush is unnecessary, KVM zaps everything if and only the VM
* is being destroyed or the userspace VMM has exited. In both cases,
--
2.35.1.574.g5d30c73bfb-goog