Re: [PATCH] KVM: x86/mmu: Use try_cmpxchg64 in tdp_mmu_set_spte_atomic

[David Matlack <dmatlack@google.com>]

On Wed, May 18, 2022 at 7:08 AM Uros Bizjak <ubizjak@gmail.com> wrote:
>
> 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>

Nice cleanup.

Reviewed-by: David Matlack <dmatlack@google.com>

> ---
> Patch requires commits 0aa7be05d83cc584da0782405e8007e351dfb6cc
> and c2df0a6af177b6c06a859806a876f92b072dc624 from tip.git
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 12 +-----------
>  1 file changed, 1 insertion(+), 11 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 922b06bf4b94..1ccc1a0f8123 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/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);
> --
> 2.35.1
>