linux-kernel - Re: [PATCH v5 01/13] KVM: x86: Cache total page count to avoid traversing the memslot array

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Date:   Wed, 20 Oct 2021 20:41:04 +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 v5 01/13] KVM: x86: Cache total page count to avoid
 traversing the memslot array

On 20.10.2021 00:24, Sean Christopherson wrote:
> On Mon, Sep 20, 2021, Maciej S. Szmigiero wrote:
>> From: "Maciej S. Szmigiero" <maciej.szmigiero@...cle.com>
>>
>> There is no point in recalculating from scratch the total number of pages
>> in all memslots each time a memslot is created or deleted.
>>
>> Just cache the value and update it accordingly on each such operation so
>> the code doesn't need to traverse the whole memslot array each time.
>>
>> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@...cle.com>
>> ---
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 28ef14155726..65fdf27b9423 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -11609,9 +11609,23 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
>>   				const struct kvm_memory_slot *new,
>>   				enum kvm_mr_change change)
>>   {
>> -	if (!kvm->arch.n_requested_mmu_pages)
>> -		kvm_mmu_change_mmu_pages(kvm,
>> -				kvm_mmu_calculate_default_mmu_pages(kvm));
>> +	if (change == KVM_MR_CREATE)
>> +		kvm->arch.n_memslots_pages += new->npages;
>> +	else if (change == KVM_MR_DELETE) {
>> +		WARN_ON(kvm->arch.n_memslots_pages < old->npages);
>> +		kvm->arch.n_memslots_pages -= old->npages;
>> +	}
>> +
>> +	if (!kvm->arch.n_requested_mmu_pages) {
(..)
>> +		u64 memslots_pages;
>> +		unsigned long nr_mmu_pages;
>> +
>> +		memslots_pages = kvm->arch.n_memslots_pages * KVM_PERMILLE_MMU_PAGES;
>> +		do_div(memslots_pages, 1000);
>> +		nr_mmu_pages = max_t(typeof(nr_mmu_pages),
>> +				     memslots_pages, KVM_MIN_ALLOC_MMU_PAGES);
> 
> "memslots_pages" is a bit of a misnomer.  Any objection to avoiding naming problems
> by explicitly casting to an "unsigned long" and simply operating on nr_mmu_pages?
> 
> 		nr_mmu_pages = (unsigned long)kvm->arch.n_memslots_pages;
> 		nr_mmu_pages *= (KVM_PERMILLE_MMU_PAGES / 1000);
> 		nr_mmu_pages = max(nr_mmu_pages, KVM_MIN_ALLOC_MMU_PAGES);
> 		kvm_mmu_change_mmu_pages(kvm, nr_mmu_pages);
> 
> E.g. the whole thing can be
> 
> 	if (!kvm->arch.n_requested_mmu_pages &&
> 	    (change == KVM_MR_CREATE || change == KVM_MR_DELETE)) {
> 		unsigned long nr_mmu_pages;
> 
> 		if (change == KVM_MR_CREATE) {
> 			kvm->arch.n_memslots_pages += new->npages;
> 		} else {
> 			WARN_ON(kvm->arch.n_memslots_pages < old->npages);
> 			kvm->arch.n_memslots_pages -= old->npages;
> 		}
> 
> 		nr_mmu_pages = (unsigned long)kvm->arch.n_memslots_pages;
> 		nr_mmu_pages *= (KVM_PERMILLE_MMU_PAGES / 1000);

The above line will set nr_mmu_pages to zero since KVM_PERMILLE_MMU_PAGES
is 20, so when integer-divided by 1000 will result in a multiplication
coefficient of zero.

> 		nr_mmu_pages = max(nr_mmu_pages, KVM_MIN_ALLOC_MMU_PAGES);
> 		kvm_mmu_change_mmu_pages(kvm, nr_mmu_pages);
> 	}
> 
>> +		kvm_mmu_change_mmu_pages(kvm, nr_mmu_pages);
>> +	}
>>   
>>   	kvm_mmu_slot_apply_flags(kvm, old, new, change);
>>   

Thanks,
Maciej