KVM: x86/mmu: Remove spurious TLB flush from TDP MMU's change_pte() hook

Remove an unnecessary remote TLB flush from set_tdp_spte(), the TDP MMu's
hook for handling change_pte() invocations from the MMU notifier.  If
the new host PTE is writable, the flush is completely redundant as there
are no futher changes to the SPTE before the post-loop flush.  If the
host PTE is read-only, then the primary MMU is responsible for ensuring
that the contents of the old and new pages are identical, thus it's safe
to let the guest continue reading both the old and new pages.  KVM must
only ensure the old page cannot be referenced after returning from its
callback; this is handled by the post-loop flush.

Fixes: 1d8dd6b3 ("kvm: x86/mmu: Support changed pte notifier in tdp MMU")
Cc: Ben Gardon <bgardon@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210226010329.1766033-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/tdp_mmu.c

@@ -1042,10 +1042,14 @@ static int set_tdp_spte(struct kvm *kvm, struct kvm_memory_slot *slot,
 		if (!is_shadow_present_pte(iter.old_spte))
 			break;
 
+		/*
+		 * Note, when changing a read-only SPTE, it's not strictly
+		 * necessary to zero the SPTE before setting the new PFN, but
+		 * doing so preserves the invariant that the PFN of a present
+		 * leaf SPTE can never change.  See __handle_changed_spte().
+		 */
 		tdp_mmu_set_spte(kvm, &iter, 0);
 
-		kvm_flush_remote_tlbs_with_address(kvm, iter.gfn, 1);
-
 		if (!pte_write(*ptep)) {
 			new_spte = kvm_mmu_changed_pte_notifier_make_spte(
 					iter.old_spte, new_pfn);