linux-kernel - [PATCH v3 14/28] KVM: x86/mmu: Skip remote TLB flush when zapping all of TDP MMU

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Message-Id: <20220226001546.360188-15-seanjc@google.com>
Date:   Sat, 26 Feb 2022 00:15:32 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, David Matlack <dmatlack@...gle.com>,
        Ben Gardon <bgardon@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>
Subject: [PATCH v3 14/28] KVM: x86/mmu: Skip remote TLB flush when zapping all
 of TDP MMU

Don't flush the TLBs when zapping all TDP MMU pages, as the only time KVM
uses the slow version of "zap everything" is when the VM is being
destroyed or the owning mm has exited.  In either case, KVM_RUN is
unreachable for the VM, i.e. the guest TLB entries cannot be consumed.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/mmu/tdp_mmu.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index c231b60e1726..87706e9cc6f3 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -874,14 +874,15 @@ bool __kvm_tdp_mmu_zap_gfn_range(struct kvm *kvm, int as_id, gfn_t start,
 
 void kvm_tdp_mmu_zap_all(struct kvm *kvm)
 {
-	bool flush = false;
 	int i;
 
+	/*
+	 * 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,
+	 * KVM_RUN is unreachable, i.e. no vCPUs will ever service the request.
+	 */
 	for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++)
-		flush = kvm_tdp_mmu_zap_gfn_range(kvm, i, 0, -1ull, flush);
-
-	if (flush)
-		kvm_flush_remote_tlbs(kvm);
+		(void)kvm_tdp_mmu_zap_gfn_range(kvm, i, 0, -1ull, false);
 }
 
 /*
-- 
2.35.1.574.g5d30c73bfb-goog