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Date:   Wed, 18 Aug 2021 16:29:10 +0200
From:   Claudio Imbrenda <imbrenda@linux.ibm.com>
To:     "Maciej S. Szmigiero" <mail@maciej.szmigiero.name>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Igor Mammedov <imammedo@redhat.com>,
        Marc Zyngier <maz@kernel.org>,
        James Morse <james.morse@arm.com>,
        Julien Thierry <julien.thierry.kdev@gmail.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Huacai Chen <chenhuacai@kernel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
        Paul Mackerras <paulus@ozlabs.org>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        David Hildenbrand <david@redhat.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 05/13] KVM: Integrate gfn_to_memslot_approx() into
 search_memslots()
Message-ID: <20210818162910.700c16d0@p-imbrenda>
In-Reply-To: <278846ca7b51ab945bdaf1418f8ca94572f228d6.1628871413.git.maciej.szmigiero@oracle.com>
References: <cover.1628871411.git.maciej.szmigiero@oracle.com>
        <278846ca7b51ab945bdaf1418f8ca94572f228d6.1628871413.git.maciej.szmigiero@oracle.com>
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On Fri, 13 Aug 2021 21:33:18 +0200
"Maciej S. Szmigiero" <mail@maciej.szmigiero.name> wrote:

> From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
> 
> s390 arch has gfn_to_memslot_approx() which is almost identical to
> search_memslots(), differing only in that in case the gfn falls in a hole
> one of the memslots bordering the hole is returned.
> 
> Add this lookup mode as an option to search_memslots() so we don't have two
> almost identical functions for looking up a memslot by its gfn.
> 
> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>

> ---
>  arch/s390/kvm/kvm-s390.c | 39 ++-------------------------------------
>  include/linux/kvm_host.h | 25 ++++++++++++++++++++++---
>  virt/kvm/kvm_main.c      |  2 +-
>  3 files changed, 25 insertions(+), 41 deletions(-)
> 
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index 4bed65dbad5e..6b3f05086fae 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -1941,41 +1941,6 @@ static long kvm_s390_set_skeys(struct kvm *kvm, struct kvm_s390_skeys *args)
>  /* for consistency */
>  #define KVM_S390_CMMA_SIZE_MAX ((u32)KVM_S390_SKEYS_MAX)
>  
> -/*
> - * Similar to gfn_to_memslot, but returns the index of a memslot also when the
> - * address falls in a hole. In that case the index of one of the memslots
> - * bordering the hole is returned.
> - */
> -static int gfn_to_memslot_approx(struct kvm_memslots *slots, gfn_t gfn)
> -{
> -	int start = 0, end = slots->used_slots;
> -	int slot = atomic_read(&slots->last_used_slot);
> -	struct kvm_memory_slot *memslots = slots->memslots;
> -
> -	if (gfn >= memslots[slot].base_gfn &&
> -	    gfn < memslots[slot].base_gfn + memslots[slot].npages)
> -		return slot;
> -
> -	while (start < end) {
> -		slot = start + (end - start) / 2;
> -
> -		if (gfn >= memslots[slot].base_gfn)
> -			end = slot;
> -		else
> -			start = slot + 1;
> -	}
> -
> -	if (start >= slots->used_slots)
> -		return slots->used_slots - 1;
> -
> -	if (gfn >= memslots[start].base_gfn &&
> -	    gfn < memslots[start].base_gfn + memslots[start].npages) {
> -		atomic_set(&slots->last_used_slot, start);
> -	}
> -
> -	return start;
> -}
> -
>  static int kvm_s390_peek_cmma(struct kvm *kvm, struct kvm_s390_cmma_log *args,
>  			      u8 *res, unsigned long bufsize)
>  {
> @@ -2002,8 +1967,8 @@ static int kvm_s390_peek_cmma(struct kvm *kvm, struct kvm_s390_cmma_log *args,
>  static unsigned long kvm_s390_next_dirty_cmma(struct kvm_memslots *slots,
>  					      unsigned long cur_gfn)
>  {
> -	int slotidx = gfn_to_memslot_approx(slots, cur_gfn);
> -	struct kvm_memory_slot *ms = slots->memslots + slotidx;
> +	struct kvm_memory_slot *ms = __gfn_to_memslot_approx(slots, cur_gfn, true);
> +	int slotidx = ms - slots->memslots;
>  	unsigned long ofs = cur_gfn - ms->base_gfn;
>  
>  	if (ms->base_gfn + ms->npages <= cur_gfn) {
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 2d62581b400e..6d0bbd6c8554 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1232,10 +1232,14 @@ try_get_memslot(struct kvm_memslots *slots, int slot_index, gfn_t gfn)
>   * Returns a pointer to the memslot that contains gfn and records the index of
>   * the slot in index. Otherwise returns NULL.
>   *
> + * With "approx" set returns the memslot also when the address falls
> + * in a hole. In that case one of the memslots bordering the hole is
> + * returned.
> + *
>   * IMPORTANT: Slots are sorted from highest GFN to lowest GFN!
>   */
>  static inline struct kvm_memory_slot *
> -search_memslots(struct kvm_memslots *slots, gfn_t gfn, int *index)
> +search_memslots(struct kvm_memslots *slots, gfn_t gfn, int *index, bool approx)
>  {
>  	int start = 0, end = slots->used_slots;
>  	struct kvm_memory_slot *memslots = slots->memslots;
> @@ -1253,11 +1257,20 @@ search_memslots(struct kvm_memslots *slots, gfn_t gfn, int *index)
>  			start = slot + 1;
>  	}
>  
> +	if (approx && start >= slots->used_slots) {
> +		*index = slots->used_slots - 1;
> +		return &memslots[slots->used_slots - 1];
> +	}
> +
>  	slot = try_get_memslot(slots, start, gfn);
>  	if (slot) {
>  		*index = start;
>  		return slot;
>  	}
> +	if (approx) {
> +		*index = start;
> +		return &memslots[start];
> +	}
>  
>  	return NULL;
>  }
> @@ -1268,7 +1281,7 @@ 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)
>  {
>  	struct kvm_memory_slot *slot;
>  	int slot_index = atomic_read(&slots->last_used_slot);
> @@ -1277,7 +1290,7 @@ __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;
> @@ -1286,6 +1299,12 @@ __gfn_to_memslot(struct kvm_memslots *slots, gfn_t gfn)
>  	return NULL;
>  }
>  
> +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)
>  {
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 207306f7c559..03ef42d2e421 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2074,7 +2074,7 @@ struct kvm_memory_slot *kvm_vcpu_gfn_to_memslot(struct kvm_vcpu *vcpu, gfn_t gfn
>  	 * search_memslots() instead of __gfn_to_memslot() to avoid
>  	 * thrashing the VM-wide last_used_index in kvm_memslots.
>  	 */
> -	slot = search_memslots(slots, gfn, &slot_index);
> +	slot = search_memslots(slots, gfn, &slot_index, false);
>  	if (slot) {
>  		vcpu->last_used_slot = slot_index;
>  		return slot;