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

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Date:   Wed, 3 Feb 2021 14:52:42 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Paolo Bonzini <pbonzini@...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 14:46, Paolo Bonzini wrote:
> On 03/02/21 14:44, David Hildenbrand wrote:
>> BTW: what are your thoughts regarding converting the rmap array on
>> x86-64 into some dynamic datastructre (xarray etc)? Has that already
>> been discussed?
> 
> Hasn't been discussed---as always, showing the code would be the best
> way to start a discussion. :)

If only a workday would have more hours :)

> 
> 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 :) )

-- 
Thanks,

David / dhildenb