linux-kernel - Re: [PATCH] KVM: Use consistent type for return value of kvm_mmu_memory_cache_nr_free

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Date:   Tue, 14 Jun 2022 09:31:23 -0700
From:   David Matlack <dmatlack@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Bo Liu <liubo03@...pur.com>, Paolo Bonzini <pbonzini@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: Use consistent type for return value of kvm_mmu_memory_cache_nr_free_objects()

On Tue, Jun 14, 2022 at 8:01 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Tue, Jun 14, 2022, Bo Liu wrote:
> > The return value type of the function rmap_can_add() is "bool", and it will
> > returns the result of the function kvm_mmu_memory_cache_nr_free_objects().
> > So we should change the return value type of
> > kvm_mmu_memory_cache_nr_free_objects() to "bool".
> >
> > Signed-off-by: Bo Liu <liubo03@...pur.com>
> > ---
> >  include/linux/kvm_host.h | 2 +-
> >  virt/kvm/kvm_main.c      | 4 ++--
> >  2 files changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index c20f2d55840c..a399a7485795 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -1358,7 +1358,7 @@ void kvm_flush_remote_tlbs(struct kvm *kvm);
> >
> >  #ifdef KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE
> >  int kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int min);
> > -int kvm_mmu_memory_cache_nr_free_objects(struct kvm_mmu_memory_cache *mc);
> > +bool kvm_mmu_memory_cache_nr_free_objects(struct kvm_mmu_memory_cache *mc);
> >  void kvm_mmu_free_memory_cache(struct kvm_mmu_memory_cache *mc);
> >  void *kvm_mmu_memory_cache_alloc(struct kvm_mmu_memory_cache *mc);
> >  #endif
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index a67e996cbf7f..2872569e3580 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -394,9 +394,9 @@ int kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int min)
> >       return 0;
> >  }
> >
> > -int kvm_mmu_memory_cache_nr_free_objects(struct kvm_mmu_memory_cache *mc)
> > +bool kvm_mmu_memory_cache_nr_free_objects(struct kvm_mmu_memory_cache *mc)
>
> Absolutely not, the name of the function is "nr_free_objects".  Renaming it to
> "has_free_objects" is not a net positive IMO.

Also nested eager page splitting [1] uses
kvm_mmu_memory_cache_nr_free_objects() and needs to know the number of
free objects.

[1] https://lore.kernel.org/all/20220516232138.1783324-23-dmatlack@google.com/

> If we really care about returning
> a bool then we can tweak rmap_can_add().
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 17252f39bd7c..047855d134da 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -1018,7 +1018,7 @@ static bool rmap_can_add(struct kvm_vcpu *vcpu)
>         struct kvm_mmu_memory_cache *mc;
>
>         mc = &vcpu->arch.mmu_pte_list_desc_cache;
> -       return kvm_mmu_memory_cache_nr_free_objects(mc);
> +       return !!kvm_mmu_memory_cache_nr_free_objects(mc);
>  }
>
>  static void rmap_remove(struct kvm *kvm, u64 *spte)