linux-kernel - Re: [PATCH v4 00/19] KVM: Dynamically size memslot arrays

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Date:   Wed, 18 Dec 2019 18:10:47 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     James Hogan <jhogan@...nel.org>,
        Paul Mackerras <paulus@...abs.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        <linux-mips@...r.kernel.org>, <kvm-ppc@...r.kernel.org>,
        <kvm@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>,
        <kvmarm@...ts.cs.columbia.edu>, <linux-kernel@...r.kernel.org>,
        Christoffer Dall <christoffer.dall@....com>,
        Philippe Mathieu-Daudé <f4bug@...at.org>
Subject: Re: [PATCH v4 00/19] KVM: Dynamically size memslot arrays

On 2019-12-17 20:40, Sean Christopherson wrote:
> The end goal of this series is to dynamically size the memslot array 
> so
> that KVM allocates memory based on the number of memslots in use, as
> opposed to unconditionally allocating memory for the maximum number 
> of
> memslots.  On x86, each memslot consumes 88 bytes, and so with 2 
> address
> spaces of 512 memslots, each VM consumes ~90k bytes for the memslots.
> E.g. given a VM that uses a total of 30 memslots, dynamic sizing 
> reduces
> the memory footprint from 90k to ~2.6k bytes.
>
> The changes required to support dynamic sizing are relatively small,
> e.g. are essentially contained in patches 17/19 and 18/19.
>
> Patches 2-16 clean up the memslot code, which has gotten quite 
> crusty,
> especially __kvm_set_memory_region().  The clean up is likely not 
> strictly
> necessary to switch to dynamic sizing, but I didn't have a remotely
> reasonable level of confidence in the correctness of the dynamic 
> sizing
> without first doing the clean up.
>
> The only functional change in v4 is the addition of an x86-specific 
> bug
> fix in x86's handling of KVM_MR_MOVE.  The bug fix is not directly 
> related
> to dynamically allocating memslots, but it has subtle and hidden 
> conflicts
> with the cleanup patches, and the fix is higher priority than 
> anything
> else in the series, i.e. should be merged first.
>
> On non-x86 architectures, v3 and v4 should be functionally 
> equivalent,
> the only non-x86 change in v4 is the dropping of a "const" in
> kvm_arch_commit_memory_region().

Gave it another go on top of 5.5-rc2 on an arm64 box, and nothing
exploded. So thumbs up from me.

Thanks,

        M.
-- 
Jazz is not dead. It just smells funny...