[PATCH v4 15/30] KVM: x86/mmu: Skip remote TLB flush when zapping all of TDP MMU

[Paolo Bonzini]

From: Sean Christopherson <seanjc@xxxxxxxxxx>

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@xxxxxxxxxx>
Reviewed-by: Ben Gardon <bgardon@xxxxxxxxxx>
Message-Id: <20220226001546.360188-15-seanjc@xxxxxxxxxx>
Reviewed-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 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 dc9db5057f3b..f59f3ff5cb75 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -860,14 +860,15 @@ 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);
 }
 
 static struct kvm_mmu_page *next_invalidated_root(struct kvm *kvm,
-- 
2.31.1