From:   "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
Subject: Re: [PATCH v5.5 30/30] KVM: Dynamically allocate "new" memslots from
 the get-go
To:     Sean Christopherson <seanjc@google.com>
Cc:     James Morse <james.morse@arm.com>,
        Alexandru Elisei <alexandru.elisei@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Atish Patra <atish.patra@wdc.com>,
        David Hildenbrand <david@redhat.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu,
        linux-mips@vger.kernel.org, kvm@vger.kernel.org,
        kvm-ppc@vger.kernel.org, kvm-riscv@lists.infradead.org,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        Ben Gardon <bgardon@google.com>,
        Huacai Chen <chenhuacai@kernel.org>,
        Marc Zyngier <maz@kernel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
        Paul Mackerras <paulus@ozlabs.org>,
        Anup Patel <anup.patel@wdc.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        Paolo Bonzini <pbonzini@redhat.com>
References: <20211104002531.1176691-1-seanjc@google.com>
 <20211104002531.1176691-31-seanjc@google.com>
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On 04.11.2021 01:25, Sean Christopherson wrote:
> Allocate the "new" memslot for !DELETE memslot updates straight away
> instead of filling an intermediate on-stack object and forcing
> kvm_set_memslot() to juggle the allocation and do weird things like reuse
> the old memslot object in MOVE.
> 
> In the MOVE case, this results in an "extra" memslot allocation due to
> allocating both the "new" slot and the "invalid" slot, but that's a
> temporary and not-huge allocation, and MOVE is a relatively rare memslot
> operation.
> 
> Regarding MOVE, drop the open-coded management of the gfn tree with a
> call to kvm_replace_memslot(), which already handles the case where
> new->base_gfn != old->base_gfn.  This is made possible by virtue of not
> having to copy the "new" memslot data after erasing the old memslot from
> the gfn tree.  Using kvm_replace_memslot(), and more specifically not
> reusing the old memslot, means the MOVE case now does hva tree and hash
> list updates, but that's a small price to pay for simplifying the code
> and making MOVE align with all the other flavors of updates.  The "extra"
> updates are firmly in the noise from a performance perspective, e.g. the
> "move (in)active area" selfttests show a (very, very) slight improvement.
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>

For a new patch set version when the "main" commit is rewritten anyway
(I mean the one titled "Keep memslots in tree-based structures instead of
array-based ones") it makes sense to integrate changes like these into
such modified main commit.

This way a full algorithm / logic check for all the supported memslot
operations needs to be done only once instead of having to be done
multiple times for all these intermediate forms of the code (as this is
a quite time-consuming job to do properly).

I think it only makes sense to separate non-functional changes (like
renaming of variables, comment rewording, open-coding a helper, etc.)
into their own patches for ease of reviewing.

Or if the main commit was unchanged from the last reviewed version so
actual changes in the new version will stand out.

Thanks,
Maciej