linux-kernel - Re: [PATCH v3 2/8] KVM: Integrate gfn_to_memslot_approx() into search

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Message-ID: <YKWB9bPyyFfo0uhf@google.com>
Date:   Wed, 19 May 2021 21:24:05 +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>,
        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>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/8] KVM: Integrate gfn_to_memslot_approx() into
 search_memslots()

On Sun, May 16, 2021, Maciej S. Szmigiero wrote:
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 8895b95b6a22..3c40c7d32f7e 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1091,10 +1091,14 @@ bool kvm_arch_irqfd_allowed(struct kvm *kvm, struct kvm_irqfd *args);
>   * gfn_to_memslot() itself isn't here as an inline because that would
>   * bloat other code too much.
>   *
> + * With "approx" set returns the memslot also when the address falls
> + * in a hole. In that case one of the memslots bordering the hole is
> + * returned.
> + *
>   * IMPORTANT: Slots are sorted from highest GFN to lowest GFN!
>   */
>  static inline struct kvm_memory_slot *
> -search_memslots(struct kvm_memslots *slots, gfn_t gfn)
> +search_memslots(struct kvm_memslots *slots, gfn_t gfn, bool approx)

An alternative to modifying the PPC code would be to make the existing
search_memslots() a wrapper to __search_memslots(), with the latter taking
@approx.

We might also want to make this __always_inline to improve the likelihood of the
compiler optimizing away @approx.  I doubt it matters in practice...

>  {
>  	int start = 0, end = slots->used_slots;
>  	int slot = atomic_read(&slots->lru_slot);
> @@ -1116,19 +1120,22 @@ search_memslots(struct kvm_memslots *slots, gfn_t gfn)
>  			start = slot + 1;
>  	}
>  
> +	if (approx && start >= slots->used_slots)
> +		return &memslots[slots->used_slots - 1];
> +
>  	if (start < slots->used_slots && gfn >= memslots[start].base_gfn &&
>  	    gfn < memslots[start].base_gfn + memslots[start].npages) {
>  		atomic_set(&slots->lru_slot, start);
>  		return &memslots[start];
>  	}
>  
> -	return NULL;
> +	return approx ? &memslots[start] : NULL;
>  }
>  
>  static inline struct kvm_memory_slot *
>  __gfn_to_memslot(struct kvm_memslots *slots, gfn_t gfn)
>  {
> -	return search_memslots(slots, gfn);
> +	return search_memslots(slots, gfn, false);
>  }
>  
>  static inline unsigned long