KVM: x86/mmu: Use try_cmpxchg64 in tdp_mmu_set_spte_atomic

Use try_cmpxchg64 instead of cmpxchg64 (*ptr, old, new) != old in
tdp_mmu_set_spte_atomic.  cmpxchg returns success in ZF flag, so this
change saves a compare after cmpxchg (and related move instruction
in front of cmpxchg). Also, remove explicit assignment to iter->old_spte
when cmpxchg fails, this is what try_cmpxchg does implicitly.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Reviewed-by: David Matlack <dmatlack@google.com>
Message-Id: <20220518135111.3535-1-ubizjak@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/tdp_mmu.c

@@ -633,7 +633,6 @@ static inline int tdp_mmu_set_spte_atomic(struct kvm *kvm,
 					  u64 new_spte)
 {
 	u64 *sptep = rcu_dereference(iter->sptep);
-	u64 old_spte;
 
 	/*
 	 * The caller is responsible for ensuring the old SPTE is not a REMOVED
@@ -649,17 +648,8 @@ static inline int tdp_mmu_set_spte_atomic(struct kvm *kvm,
 	 * Note, fast_pf_fix_direct_spte() can also modify TDP MMU SPTEs and
 	 * does not hold the mmu_lock.
 	 */
-	old_spte = cmpxchg64(sptep, iter->old_spte, new_spte);
-	if (old_spte != iter->old_spte) {
-		/*
-		 * The page table entry was modified by a different logical
-		 * CPU. Refresh iter->old_spte with the current value so the
-		 * caller operates on fresh data, e.g. if it retries
-		 * tdp_mmu_set_spte_atomic().
-		 */
-		iter->old_spte = old_spte;
+	if (!try_cmpxchg64(sptep, &iter->old_spte, new_spte))
 		return -EBUSY;
-	}
 
 	__handle_changed_spte(kvm, iter->as_id, iter->gfn, iter->old_spte,
 			      new_spte, iter->level, true);