Re: [PATCH 20/24] kvm: x86/mmu: Add atomic option for setting SPTEs

Paolo Bonzini
2021-01-26T14:23:53Z

On 12/01/21 19:10, Ben Gardon wrote:
> static void handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
> - u64 old_spte, u64 new_spte, int level);
> + u64 old_spte, u64 new_spte, int level,
> + bool atomic);

If you don't mind, I prefer "shared" as the name for the new argument
(i.e. "this is what you need to know", rathar than "this is what I want
you to do").

>
> +/*
> + * tdp_mmu_set_spte_atomic - Set a TDP MMU SPTE atomically and handle the
> + * associated bookkeeping
> + *
> + * @kvm: kvm instance
> + * @iter: a tdp_iter instance currently on the SPTE that should be set
> + * @new_spte: The value the SPTE should be set to
> + * Returns: true if the SPTE was set, false if it was not. If false is returned,
> + * this function will have no side-effects.
> + */
> +static inline bool tdp_mmu_set_spte_atomic(struct kvm *kvm,
> + struct tdp_iter *iter,
> + u64 new_spte)
> +{
> + u64 *root_pt = tdp_iter_root_pt(iter);
> + struct kvm_mmu_page *root = sptep_to_sp(root_pt);
> + int as_id = kvm_mmu_page_as_id(root);
> +
> + kvm_mmu_lock_assert_held_shared(kvm);
> +
> + if (cmpxchg64(iter->sptep, iter->old_spte, new_spte) != iter->old_spte)
> + return false;
> +
> + handle_changed_spte(kvm, as_id, iter->gfn, iter->old_spte, new_spte,
> + iter->level, true);
> +
> + return true;
> +}
> +
> +

Still unused as of this patch, so please move it where it's used.

Note that in this case, "atomic" in the name is appropriate, think of
hypothetical code like this:

if (!shared)
tdp_mmu_set_spte(...)
else if (!tdp_mmu_set_spte_atomic(...)

which says "if there could be concurrent changes, be careful and do
everything with atomic operations".

Paolo