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

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Date:   Tue, 19 Oct 2021 23:38:50 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
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 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