linux-kernel - Re: [PATCH 2/2] KVM: Scalable memslots implementation

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Date:   Wed, 3 Feb 2021 15:33:38 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Hildenbrand <david@...hat.com>,
        "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     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>,
        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 2/2] KVM: Scalable memslots implementation

On 03/02/21 14:52, David Hildenbrand wrote:
>>
>> However, note that the TDP MMU does not need an rmap at all.  Since that
>> one is getting ready to become the default, the benefits of working on
>> the rmap would be quite small and only affect nested virtualization.
> 
> Right, but we currently always have to allocate it.
> 
> 8 bytes per 4k page, 8 bytes per 2M page, 8 bytes per 1G page.
> 
> The 4k part alone is 0.2% of the memblock size.
> 
> For a 1 TB memslot we might "waste" > 2 GB on rmap arrays.
> 
> (that's why I am asking :) )

Yes, we're in violent agreement. :)  I mean that the simplest solution 
would be to allocate it lazily when building a non-TDP root.

Paolo