From: Sean Christopherson <seanjc@xxxxxxxxxx>
Remove the misleading flush "handling" when zapping invalidated TDP MMU
roots, and document that flushing is unnecessary for all flavors of MMUs
when zapping invalid/obsolete roots/pages. The "handling" in the TDP MMU
is dead code, as zap_gfn_range() is called with shared=true, in which
case it will never return true due to the flushing being handled by
tdp_mmu_zap_spte_atomic().
No functional change intended.
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Reviewed-by: Ben Gardon <bgardon@xxxxxxxxxx>
Message-Id: <20220226001546.360188-6-seanjc@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
arch/x86/kvm/mmu/mmu.c | 10 +++++++---
arch/x86/kvm/mmu/tdp_mmu.c | 15 ++++++++++-----
2 files changed, 17 insertions(+), 8 deletions(-)
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 9a6df2d02777..8408d7db8d2a 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -5674,9 +5674,13 @@ static void kvm_zap_obsolete_pages(struct kvm *kvm)
}
/*
- * Trigger a remote TLB flush before freeing the page tables to ensure
- * KVM is not in the middle of a lockless shadow page table walk, which
- * may reference the pages.
+ * Kick all vCPUs (via remote TLB flush) before freeing the page tables
+ * to ensure KVM is not in the middle of a lockless shadow page table
+ * walk, which may reference the pages. The remote TLB flush itself is
+ * not required and is simply a convenient way to kick vCPUs as needed.
+ * KVM performs a local TLB flush when allocating a new root (see
+ * kvm_mmu_load()), and the reload in the caller ensure no vCPUs are
+ * running with an obsolete MMU.
*/
kvm_mmu_commit_zap_page(kvm, &kvm->arch.zapped_obsolete_pages);
}
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 921fa386df99..2ce6915b70fe 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -851,7 +851,6 @@ void kvm_tdp_mmu_zap_invalidated_roots(struct kvm *kvm)
{
struct kvm_mmu_page *next_root;
struct kvm_mmu_page *root;
- bool flush = false;
lockdep_assert_held_read(&kvm->mmu_lock);
@@ -864,7 +863,16 @@ void kvm_tdp_mmu_zap_invalidated_roots(struct kvm *kvm)
rcu_read_unlock();
- flush = zap_gfn_range(kvm, root, 0, -1ull, true, flush, true);
+ /*
+ * A TLB flush is unnecessary, invalidated roots are guaranteed
+ * to be unreachable by the guest (see kvm_tdp_mmu_put_root()
+ * for more details), and unlike the legacy MMU, no vCPU kick
+ * is needed to play nice with lockless shadow walks as the TDP
+ * MMU protects its paging structures via RCU. Note, zapping
+ * will still flush on yield, but that's a minor performance
+ * blip and not a functional issue.
+ */
+ (void)zap_gfn_range(kvm, root, 0, -1ull, true, false, true);
/*
* Put the reference acquired in
@@ -878,9 +886,6 @@ void kvm_tdp_mmu_zap_invalidated_roots(struct kvm *kvm)
}
rcu_read_unlock();
-
- if (flush)
- kvm_flush_remote_tlbs(kvm);
}
/*
--
2.31.1
