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

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Message-ID: <4bdcb44c-c35d-45b2-c0c1-e857e0fd383e@redhat.com>
Date:   Wed, 3 Feb 2021 14:41:02 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "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>,
        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 2/2] KVM: Scalable memslots implementation

On 02/02/21 23:42, Maciej S. Szmigiero wrote:
>> I'm not opposed to using more sophisticated storage for the gfn 
>> lookups, but only if there's a good reason for doing so.  IMO, the
>> rbtree isn't simpler, just different.

And it also has worse cache utilization than an array, due to memory 
footprint (as you point out below) but also pointer chasing.

>> Memslot modifications are
>> unlikely to be a hot path (and if it is, x86's "zap everything"
>> implementation is a far bigger problem), and it's hard to beat the
>> memory footprint of a raw array.  That doesn't leave much 
>> motivation for such a big change to some of KVM's scariest (for me)
>> code.
>> 
> 
> Improvements can be done step-by-step, 
> kvm_mmu_invalidate_zap_pages_in_memslot() can be rewritten, too in
> the future, if necessary. After all, complains are that this change
> alone is too big.
> 
> I think that if you look not at the patch itself but at the
> resulting code the new implementation looks rather straightforward,
> there are comments at every step in kvm_set_memslot() to explain
> exactly what is being done. Not only it already scales well, but it
> is also flexible to accommodate further improvements or even new
> operations.
> 
> The new implementation also uses standard kernel {rb,interval}-tree
> and hash table implementation as its basic data structures, so it 
> automatically benefits from any generic improvements to these.
> 
> All for the low price of just 174 net lines of code added.

I think the best thing to do here is to provide a patch series that 
splits the individual changes so that they can be reviewed and their 
separate merits evaluated.

Another thing that I dislike about KVM_SET_USER_MEMORY_REGION is that 
IMO userspace should provide all memslots at once, for an atomic switch 
of the whole memory array.  (Or at least I would like to see the code; 
it might be a bit tricky because you'll need code to compute the 
difference between the old and new arrays and invoke 
kvm_arch_prepare/commit_memory_region).  I'm not sure how that would 
interact with the active/inactive pair that you introduce here.

Paolo