linux-kernel - Re: [PATCH 3/3] KVM: x86/mmu: Stop zapping invalidated TDP MMU roots asynchronously

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Message-ID: <9ebf281f-7379-ba37-2867-8918da813c19@redhat.com>
Date:   Thu, 21 Sep 2023 12:02:33 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Pattara Teerapong <pteerapong@...gle.com>,
        David Stevens <stevensd@...gle.com>,
        Yiwei Zhang <zzyiwei@...gle.com>,
        Paul Hsia <paulhsia@...gle.com>
Subject: Re: [PATCH 3/3] KVM: x86/mmu: Stop zapping invalidated TDP MMU roots
 asynchronously

On 9/16/23 02:39, Sean Christopherson wrote:
> +		if (!root->tdp_mmu_scheduled_root_to_zap)
> +			continue;
> +
> +		root->tdp_mmu_scheduled_root_to_zap = false;

This is protected by slots_lock... tricky.

Worth squashing in a comment and also a small update to another comment:

diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
index 93b9d50c24ad..decc1f153669 100644
--- a/arch/x86/kvm/mmu/mmu_internal.h
+++ b/arch/x86/kvm/mmu/mmu_internal.h
@@ -60,6 +60,8 @@ struct kvm_mmu_page {
  	bool unsync;
  	union {
  		u8 mmu_valid_gen;
+
+		/* Only accessed under slots_lock.  */
  		bool tdp_mmu_scheduled_root_to_zap;
  	};
  
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index ca3304c2c00c..070ee5b2c271 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -246,7 +246,7 @@ hpa_t kvm_tdp_mmu_get_vcpu_root_hpa(struct kvm_vcpu *vcpu)
  	 * by a memslot update or by the destruction of the VM.  Initialize the
  	 * refcount to two; one reference for the vCPU, and one reference for
  	 * the TDP MMU itself, which is held until the root is invalidated and
-	 * is ultimately put by tdp_mmu_zap_root_work().
+	 * is ultimately put by kvm_tdp_mmu_zap_invalidated_roots().
  	 */
  	refcount_set(&root->tdp_mmu_root_count, 2);
  

Paolo

> +		KVM_BUG_ON(!root->role.invalid, kvm);