linux-kernel - Re: [PATCH v5.5 24/30] KVM: Use interval tree to do fast hva lookup in memslots

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Date:   Fri, 12 Nov 2021 01:05:27 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Maciej S. Szmigiero" <maciej.szmigiero@...cle.com>
Cc:     James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Atish Patra <atish.patra@....com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, kvm-riscv@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Ben Gardon <bgardon@...gle.com>, Marc Zyngier <maz@...nel.org>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Anup Patel <anup.patel@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v5.5 24/30] KVM: Use interval tree to do fast hva lookup
 in memslots

On Fri, Nov 12, 2021, Maciej S. Szmigiero wrote:
> On 04.11.2021 01:25, Sean Christopherson wrote:
> > @@ -1262,22 +1274,32 @@ static void kvm_replace_memslot(struct kvm_memslots *slots,
> >   				struct kvm_memory_slot *new)
> >   {
> >   	/*
> > -	 * Remove the old memslot from the hash list, copying the node data
> > -	 * would corrupt the list.
> > +	 * Remove the old memslot from the hash list and interval tree, copying
> > +	 * the node data would corrupt the structures.
> >   	 */
> >   	if (old) {
> >   		hash_del(&old->id_node);
> > +		interval_tree_remove(&old->hva_node, &slots->hva_tree);
> >   		if (!new)
> >   			return;
> >   	}
> > -	/* Copy the source *data*, not the pointer, to the destination. */
> > -	if (old)
> > +	/*
> > +	 * Copy the source *data*, not the pointer, to the destination.  If
> > +	 * @old is NULL, initialize @new's hva range.
> > +	 */
> > +	if (old) {
> >   		*new = *old;
> > +	} else if (new) {
> 
> Unnecessary check - if "new" is NULL then the code will crash anyway
> accessing this pointer unconditionally...
> 
> > +		new->hva_node.start = new->userspace_addr;
> > +		new->hva_node.last = new->userspace_addr +
> > +				     (new->npages << PAGE_SHIFT) - 1;
> > +	}
> >   	/* (Re)Add the new memslot. */
> >   	hash_add(slots->id_hash, &new->id_node, new->id);
> > +	interval_tree_insert(&new->hva_node, &slots->hva_tree);
> 
> ...in these two lines above.

Yep, definitely worthless.  I think this was another "plan for the future" idea
that didn't actually add value.