[PATCH v2 12/13] KVM: x86/mmu: Fast invalidation for TDP MMU

Ben Gardon
2021-04-01T23:38:57Z

Provide a real mechanism for fast invalidation by marking roots as
invalid so that their reference count will quickly fall to zero
and they will be torn down.

One negative side affect of this approach is that a vCPU thread will
likely drop the last reference to a root and be saddled with the work of
tearing down an entire paging structure. This issue will be resolved in
a later commit.

Signed-off-by: Ben Gardon <bgardon@google.com>
---

Changelog
v2:
-- open code root invalidation

arch/x86/kvm/mmu/mmu.c | 26 +++++++++++++++++++++++---
arch/x86/kvm/mmu/tdp_mmu.h | 3 +++
2 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index a3837f8ad4ed..ba0c65076200 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -5418,6 +5418,8 @@ static void kvm_zap_obsolete_pages(struct kvm *kvm)
*/
static void kvm_mmu_zap_all_fast(struct kvm *kvm)
{
+ struct kvm_mmu_page *root;
+
lockdep_assert_held(&kvm->slots_lock);

write_lock(&kvm->mmu_lock);
@@ -5432,6 +5434,27 @@ static void kvm_mmu_zap_all_fast(struct kvm *kvm)
*/
kvm->arch.mmu_valid_gen = kvm->arch.mmu_valid_gen ? 0 : 1;

+
+ if (is_tdp_mmu_enabled(kvm)) {
+ /*
+ * Mark each TDP MMU root as invalid so that other threads
+ * will drop their references and allow the root count to
+ * go to 0.
+ *
+ * This has essentially the same effect for the TDP MMU
+ * as updating mmu_valid_gen above does for the shadow
+ * MMU.
+ *
+ * In order to ensure all threads see this change when
+ * handling the MMU reload signal, this must happen in the
+ * same critical section as kvm_reload_remote_mmus, and
+ * before kvm_zap_obsolete_pages as kvm_zap_obsolete_pages
+ * could drop the MMU lock and yield.
+ */
+ list_for_each_entry(root, &kvm->arch.tdp_mmu_roots, link)
+ root->role.invalid = true;
+ }
+
/*
* Notify all vcpus to reload its shadow page table and flush TLB.
* Then all vcpus will switch to new shadow page table with the new
@@ -5444,9 +5467,6 @@ static void kvm_mmu_zap_all_fast(struct kvm *kvm)

kvm_zap_obsolete_pages(kvm);

- if (is_tdp_mmu_enabled(kvm))
- kvm_tdp_mmu_zap_all(kvm);
-
write_unlock(&kvm->mmu_lock);
}

diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
index d703c6d6024a..8fa3e7421a93 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.h
+++ b/arch/x86/kvm/mmu/tdp_mmu.h
@@ -10,6 +10,9 @@ 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 *kvm,
struct kvm_mmu_page *root)
{
+ if (root->role.invalid)
+ return false;
+
return refcount_inc_not_zero(&root->tdp_mmu_root_count);
}

--
2.31.0.208.g409f899ff0-goog