linux-kernel - Re: [PATCH v5.5 23/30] KVM: Resolve memslot ID via a hash table instead of via a static array

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Date:   Fri, 12 Nov 2021 00:51:47 +0100
From:   "Maciej S. Szmigiero" <maciej.szmigiero@...cle.com>
To:     Sean Christopherson <seanjc@...gle.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 23/30] KVM: Resolve memslot ID via a hash table
 instead of via a static array

On 04.11.2021 01:25, Sean Christopherson wrote:
> From: Maciej S. Szmigiero <maciej.szmigiero@...cle.com>
> 
> Memslot ID to the corresponding memslot mappings are currently kept as
> indices in static id_to_index array.
> The size of this array depends on the maximum allowed memslot count
> (regardless of the number of memslots actually in use).
> 
> This has become especially problematic recently, when memslot count cap was
> removed, so the maximum count is now full 32k memslots - the maximum
> allowed by the current KVM API.
> 
> Keeping these IDs in a hash table (instead of an array) avoids this
> problem.
> 
> Resolving a memslot ID to the actual memslot (instead of its index) will
> also enable transitioning away from an array-based implementation of the
> whole memslots structure in a later commit.
> 
> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@...cle.com>
> Co-developed-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   include/linux/kvm_host.h | 16 +++----
>   virt/kvm/kvm_main.c      | 96 +++++++++++++++++++++++++++++++---------
>   2 files changed, 84 insertions(+), 28 deletions(-)
> 
(..)
> @@ -1259,17 +1257,49 @@ static int kvm_alloc_dirty_bitmap(struct kvm_memory_slot *memslot)
>   	return 0;
>   }
>   
> +static void kvm_replace_memslot(struct kvm_memslots *slots,
> +				struct kvm_memory_slot *old,
> +				struct kvm_memory_slot *new)
> +{
> +	/*
> +	 * Remove the old memslot from the hash list, copying the node data
> +	 * would corrupt the list.
> +	 */
> +	if (old) {
> +		hash_del(&old->id_node);
> +
> +		if (!new)
> +			return;
> +	}
> +
> +	/* Copy the source *data*, not the pointer, to the destination. */
> +	if (old)
> +		*new = *old;

This way of writing it (that, is re-checking whether "old" is not-NULL)
suggests that it could have been set to NULL inside the previous block
(since the last check), which isn't true.

Thanks,
Maciej