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

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Message-ID: <Yaekjrr1OVrgwUic@google.com>
Date:   Wed, 1 Dec 2021 16:36:30 +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 Wed, Dec 01, 2021, Maciej S. Szmigiero wrote:
> 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.

I don't want the callers to have to calculate the slot.  It's mostly syntatic
sugar, but I really do think it improves readability.  And since the first thing
every caller will do is retrieve the slot, I see no benefit in forcing the caller
to do the work.

E.g. in the simple kvm_check_memslot_overlap() usage, iter.slot->id is intuitive
and easy to parse, whereas kvm_memslot_iter_slot(&iter)->id is slightly more
difficult to parse and raises questions about why a function call is necessary
and what the function might be doing.

static bool kvm_check_memslot_overlap(struct kvm_memslots *slots, int id,
				      gfn_t start, gfn_t end)
{
	struct kvm_memslot_iter iter;

	kvm_for_each_memslot_in_gfn_range(&iter, slots, start, end) {
		if (iter.slot->id != id)
			return true;
	}

	return false;
}

vs.

static bool kvm_check_memslot_overlap(struct kvm_memslots *slots, int id,
				      gfn_t start, gfn_t end)
{
	struct kvm_memslot_iter iter;

	kvm_for_each_memslot_in_gfn_range(&iter, slots, start, end) {
		if (kvm_memslot_iter_slot(&iter)->id != id)
			return true;
	}

	return false;
}