linux-kernel - Re: [PATCH v5 05/13] KVM: Integrate gfn_to_memslot_approx() into search

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Date:   Wed, 20 Oct 2021 20:41:26 +0200
From:   "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Igor Mammedov <imammedo@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 05/13] KVM: Integrate gfn_to_memslot_approx() into
 search_memslots()

On 20.10.2021 01:38, Sean Christopherson wrote:
> On Mon, Sep 20, 2021, Maciej S. Szmigiero wrote:
>> @@ -1267,7 +1280,7 @@ search_memslots(struct kvm_memslots *slots, gfn_t gfn, int *index)
>>    * itself isn't here as an inline because that would bloat other code too much.
>>    */
>>   static inline struct kvm_memory_slot *
>> -__gfn_to_memslot(struct kvm_memslots *slots, gfn_t gfn)
>> +__gfn_to_memslot_approx(struct kvm_memslots *slots, gfn_t gfn, bool approx)
> 
> This function name is a misnomer.  The helper is not an "approx" version, it's an
> inner helper that takes an @approx param.  Unless someone has a more clever name,
> the dreaded four underscores seems like the way to go.  Warning away users is a
> good thing in this case...
> 
>>   {
>>   	struct kvm_memory_slot *slot;
>>   	int slot_index = atomic_read(&slots->last_used_slot);
>> @@ -1276,7 +1289,7 @@ __gfn_to_memslot(struct kvm_memslots *slots, gfn_t gfn)
>>   	if (slot)
>>   		return slot;
>>   
>> -	slot = search_memslots(slots, gfn, &slot_index);
>> +	slot = search_memslots(slots, gfn, &slot_index, approx);
>>   	if (slot) {
>>   		atomic_set(&slots->last_used_slot, slot_index);
>>   		return slot;
>> @@ -1285,6 +1298,12 @@ __gfn_to_memslot(struct kvm_memslots *slots, gfn_t gfn)
>>   	return NULL;
>>   }
>>   
> 
> There's a comment that doesn't show up in this diff that should also be moved,
> and opportunistically updated.
> 
>> +static inline struct kvm_memory_slot *
>> +__gfn_to_memslot(struct kvm_memslots *slots, gfn_t gfn)
>> +{
>> +	return __gfn_to_memslot_approx(slots, gfn, false);
>> +}
>> +
>>   static inline unsigned long
>>   __gfn_to_hva_memslot(const struct kvm_memory_slot *slot, gfn_t gfn)
>>   {
> 
> E.g. this as fixup?
> 
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index 540fa948baa5..2964c773b36c 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -1964,10 +1964,15 @@ static int kvm_s390_peek_cmma(struct kvm *kvm, struct kvm_s390_cmma_log *args,
>          return 0;
>   }
> 
> +static int gfn_to_memslot_approx(struct kvm_memslots *slots, gfn_t gfn)
> +{
> +       return ____gfn_to_memslot(slots, cur_gfn, true);
> +}
> +
>   static unsigned long kvm_s390_next_dirty_cmma(struct kvm_memslots *slots,
>                                                unsigned long cur_gfn)
>   {
> -       struct kvm_memory_slot *ms = __gfn_to_memslot_approx(slots, cur_gfn, true);
> +       struct kvm_memory_slot *ms = gfn_to_memslot_approx(slots, cur_gfn);
>          int slotidx = ms - slots->memslots;
>          unsigned long ofs = cur_gfn - ms->base_gfn;
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 8fd9644f40b2..ec1a074c2f6e 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1274,13 +1274,8 @@ search_memslots(struct kvm_memslots *slots, gfn_t gfn, int *index, bool approx)
>          return NULL;
>   }
> 
> -/*
> - * __gfn_to_memslot() and its descendants are here because it is called from
> - * non-modular code in arch/powerpc/kvm/book3s_64_vio{,_hv}.c. gfn_to_memslot()
> - * itself isn't here as an inline because that would bloat other code too much.
> - */
>   static inline struct kvm_memory_slot *
> -__gfn_to_memslot_approx(struct kvm_memslots *slots, gfn_t gfn, bool approx)
> +____gfn_to_memslot(struct kvm_memslots *slots, gfn_t gfn, bool approx)
>   {
>          struct kvm_memory_slot *slot;
>          int slot_index = atomic_read(&slots->last_used_slot);
> @@ -1298,10 +1293,15 @@ __gfn_to_memslot_approx(struct kvm_memslots *slots, gfn_t gfn, bool approx)
>          return NULL;
>   }
> 
> +/*
> + * __gfn_to_memslot() and its descendants are here to allow arch code to inline
> + * the lookups in hot paths.  gfn_to_memslot() itself isn't here as an inline
> + * because that would bloat other code too much.
> + */
>   static inline struct kvm_memory_slot *
>   __gfn_to_memslot(struct kvm_memslots *slots, gfn_t gfn)
>   {
> -       return __gfn_to_memslot_approx(slots, gfn, false);
> +       return ____gfn_to_memslot(slots, gfn, false);
>   }
> 
>   static inline unsigned long
> 

Looks sensible, will apply your proposed changes.

Thanks,
Maciej