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KVM: x86/mmu: Use try_cmpxchg64 in tdp_mmu_set_spte_atomic

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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>
This commit is contained in:
Uros Bizjak 2022-05-18 15:51:11 +02:00 committed by Paolo Bonzini
parent d895f28ed6
commit aee98a6838
1 changed files with 1 additions and 11 deletions
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12
arch/x86/kvm/mmu/tdp_mmu.c
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@ -633,7 +633,6 @@ static inline int tdp_mmu_set_spte_atomic(struct kvm *kvm,
u64 new_spte) u64 new_spte)
{ {
u64 *sptep = rcu_dereference(iter->sptep); u64 *sptep = rcu_dereference(iter->sptep);
u64 old_spte;
/* /*
* The caller is responsible for ensuring the old SPTE is not a REMOVED * 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 * Note, fast_pf_fix_direct_spte() can also modify TDP MMU SPTEs and
* does not hold the mmu_lock. * does not hold the mmu_lock.
*/ */
old_spte = cmpxchg64(sptep, iter->old_spte, new_spte); if (!try_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;
return -EBUSY; return -EBUSY;
}
__handle_changed_spte(kvm, iter->as_id, iter->gfn, iter->old_spte, __handle_changed_spte(kvm, iter->as_id, iter->gfn, iter->old_spte,
new_spte, iter->level, true); new_spte, iter->level, true);