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

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Message-ID: <7119b08c-e82a-8b81-7f9e-2e79f8276d51@maciej.szmigiero.name>
Date:   Wed, 1 Dec 2021 16:45:58 +0100
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>,
        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 01.12.2021 04:41, Sean Christopherson wrote:
> 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.

"slot" can be easily calculated from "node" together with either "slots" or
"node_idx" (the code above just adjusts a pointer) so storing it in the
iterator makes little sense if the later are already stored there.

> 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.

You're right that we can get away with not storing "end", will remove it.

Thanks,
Maciej