Commit aee98a68 authored by Uros Bizjak's avatar Uros Bizjak Committed by Paolo Bonzini
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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: default avatarUros Bizjak <ubizjak@gmail.com>
Reviewed-by: default avatarDavid Matlack <dmatlack@google.com>
Message-Id: <20220518135111.3535-1-ubizjak@gmail.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent d895f28e

arch/x86/kvm/mmu/tdp_mmu.c

+1 −11
Original line number Diff line number Diff line
@@ -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);