linux-kernel - Re: [PATCH 10/13] KVM: Provide common implementation for generic dirty log functions

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Message-ID: <20190919002242.GA19503@blackberry>
Date:   Thu, 19 Sep 2019 10:22:42 +1000
From:   Paul Mackerras <paulus@...abs.org>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     James Hogan <jhogan@...nel.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Marc Zyngier <marc.zyngier@....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@....com>,
        Suzuki K Pouloze <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
Subject: Re: [PATCH 10/13] KVM: Provide common implementation for generic
 dirty log functions

On Wed, Sep 11, 2019 at 11:50:35AM -0700, Sean Christopherson wrote:
> Move the implementations of KVM_GET_DIRTY_LOG and KVM_CLEAR_DIRTY_LOG
> for CONFIG_KVM_GENERIC_DIRTYLOG_READ_PROTECT into common KVM code.
> The arch specific implemenations are extremely similar, differing
> only in whether the dirty log needs to be sync'd from hardware (x86)
> and how the TLBs are flushed.  Add new arch hooks to handle sync
> and TLB flush; the sync will also be used for non-generic dirty log
> support in a future patch (s390).
> 
> The ulterior motive for providing a common implementation is to
> eliminate the dependency between arch and common code with respect to
> the memslot referenced by the dirty log, i.e. to make it obvious in the
> code that the validity of the memslot is guaranteed, as a future patch
> will rework memslot handling such that id_to_memslot() can return NULL.

I notice you add empty definitions of kvm_arch_sync_dirty_log() for
PPC, both Book E and Book 3S.  Given that PPC doesn't select
CONFIG_KVM_GENERIC_DIRTYLOG_READ_PROTECT, why is this necessary?

Paul.