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

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Date:   Fri, 21 May 2021 09:03:43 +0200
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>,
        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 19.05.2021 23:24, Sean Christopherson wrote:
> 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.

I guess you mean that if search_memslots() only does an exact search
(like the current code does) its 3 callers won't have to be modified.
Will do it then.

> 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...

Sounds like a good idea, will do.

Thanks,
Maciej