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

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Date:   Wed, 18 Dec 2019 12:40:29 +0100
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        James Hogan <jhogan@...nel.org>,
        Paul Mackerras <paulus@...abs.org>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Marc Zyngier <maz@...nel.org>
Cc:     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 17.12.19 21:40, 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().

I gave this series a quick spin and it still seems to work on s390 (minus the selftest).