KVM: x86/mmu: Clear MMU-writable during changed_pte notifier

When handling the changed_pte notifier and the new PTE is read-only,
clear both the Host-writable and MMU-writable bits in the SPTE. This
preserves the invariant that MMU-writable is set if-and-only-if
Host-writable is set.

No functional change intended. Nothing currently relies on the
aforementioned invariant and technically the changed_pte notifier is
dead code.
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20220113233020.3986005-3-dmatlack@google.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/spte.c

@@ -215,6 +215,7 @@ u64 kvm_mmu_changed_pte_notifier_make_spte(u64 old_spte, kvm_pfn_t new_pfn)
 
 	new_spte &= ~PT_WRITABLE_MASK;
 	new_spte &= ~shadow_host_writable_mask;
+	new_spte &= ~shadow_mmu_writable_mask;
 
 	new_spte = mark_spte_for_access_track(new_spte);