linux-kernel - Re: [PATCH v6 26/29] KVM: Optimize gfn lookup in kvm_zap_gfn

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Date:   Wed, 1 Dec 2021 03:41:57 +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>,
        Joerg Roedel <joro@...tes.org>,
        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>,
        Anup Patel <anup.patel@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Atish Patra <atish.patra@....com>,
        Ben Gardon <bgardon@...gle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 26/29] KVM: Optimize gfn lookup in kvm_zap_gfn_range()

On Tue, Nov 30, 2021, Maciej S. Szmigiero wrote:
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 41efe53cf150..6fce6eb797a7 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -848,6 +848,105 @@ struct kvm_memory_slot *id_to_memslot(struct kvm_memslots *slots, int id)
>  	return NULL;
>  }
>  
> +/* Iterator used for walking memslots that overlap a gfn range. */
> +struct kvm_memslot_iter {
> +	struct kvm_memslots *slots;
> +	gfn_t end;
> +	struct rb_node *node;
> +};

...

> +static inline struct kvm_memory_slot *kvm_memslot_iter_slot(struct kvm_memslot_iter *iter)
> +{
> +	return container_of(iter->node, struct kvm_memory_slot, gfn_node[iter->slots->node_idx]);

Having to use a helper in callers of kvm_for_each_memslot_in_gfn_range() is a bit
ugly, any reason not to grab @slot as well?  Then the callers just do iter.slot,
which IMO is much more readable.

And if we do that, I'd also vote to omit slots and end from the iterator.  It would
mean passing in slots and end to kvm_memslot_iter_is_valid() and kvm_memslot_iter_next(),
but that's more idiomatic in a for-loop if iter is considered to be _just_ the iterator
part.  "slots" is arguable, but "end" really shouldn't be part of the iterator.

> +}
> +
> +static inline bool kvm_memslot_iter_is_valid(struct kvm_memslot_iter *iter)
> +{
> +	struct kvm_memory_slot *memslot;
> +
> +	if (!iter->node)
> +		return false;
> +
> +	memslot = kvm_memslot_iter_slot(iter);
> +
> +	/*
> +	 * If this slot starts beyond or at the end of the range so does
> +	 * every next one
> +	 */
> +	return memslot->base_gfn < iter->end;
> +}
> +
> +static inline void kvm_memslot_iter_next(struct kvm_memslot_iter *iter)
> +{
> +	iter->node = rb_next(iter->node);
> +}
> +
> +/* Iterate over each memslot at least partially intersecting [start, end) range */
> +#define kvm_for_each_memslot_in_gfn_range(iter, slots, start, end)	 \
> +	for (kvm_memslot_iter_start(iter, slots, start, end);		 \
> +	     kvm_memslot_iter_is_valid(iter);				 \
> +	     kvm_memslot_iter_next(iter))
> +
>  /*
>   * KVM_SET_USER_MEMORY_REGION ioctl allows the following operations:
>   * - create a new memory slot