linux-kernel - Re: [PATCH v5 12/19] KVM: Move memslot deletion to helper function

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Date:   Fri, 7 Feb 2020 10:07:20 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.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>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, Marc Zyngier <maz@...nel.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@...r.kernel.org,
        kvm-ppc@...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 v5 12/19] KVM: Move memslot deletion to helper function

On Fri, Feb 07, 2020 at 09:59:12AM -0800, Sean Christopherson wrote:
> On Thu, Feb 06, 2020 at 11:51:16AM -0500, Peter Xu wrote:
> > /*
> >  * Make a full copy of the old memslot, the pointer will become stale
> >  * when the memslots are re-sorted by update_memslots() in
> >  * kvm_delete_memslot(), while to make the kvm_free_memslot() work as
> >  * expected later on, we still need the cached memory slot.
> >  */
> 
> As above, it's more subtle than just the kvm_delete_memslot() case.
> 
> 	/*
> 	 * Make a full copy of the old memslot, the pointer will become stale
> 	 * when the memslots are re-sorted by update_memslots() when deleting
> 	 * or moving a memslot, and additional modifications to the old memslot
> 	 * need to be made after calling update_memslots().
> 	 */

Actually, that's not quite correct, as the same is true for all memslot
updates, and we still query @old after update_memslots() for CREATE and
FLAGS.  This is better.

        /*
         * Make a full copy of the old memslot, the pointer will become stale
         * when the memslots are re-sorted by update_memslots(), and the old
	 * memslot needs to be referenced after calling update_memslots(), e.g.
	 * to free its resources and for arch specific behavior.
         */