linux-kernel - Re: [PATCH v10 02/14] KVM: Cache as_id in kvm_memory

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Date:   Tue, 7 Jul 2020 12:56:58 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Andrew Jones <drjones@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Kevin Tian <kevin.tian@...el.com>
Subject: Re: [PATCH v10 02/14] KVM: Cache as_id in kvm_memory_slot

On Tue, Jul 07, 2020 at 03:50:09PM -0400, Peter Xu wrote:
> On Mon, Jul 06, 2020 at 11:17:32PM -0700, Sean Christopherson wrote:
> > On Fri, Jul 03, 2020 at 02:41:22PM -0400, Peter Xu wrote:
> > > On Thu, Jul 02, 2020 at 04:08:49PM -0700, Sean Christopherson wrote:
> > > > This technically needs to set as_id in the deleted memslot.  I highly doubt
> > > > it will ever matter from a functionality perspective, but it'd be confusing
> > > > to encounter a memslot whose as_id did not match that of its owner.
> > > 
> > > Yeah it shouldn't matter because as_id is directly passed in to look up the
> > > pointer of kvm_memslots in kvm_delete_memslot, and memslot->as_id shouldn't be
> > > further referenced.
> > > 
> > > I can add a comment above if this can clarify things a bit:
> > > 
> > > +	u16 as_id; /* cache of as_id; only valid if npages != 0 */
> > 
> > Why not just set it?
> 
> Because the value is useless even if set? :)

It's useless when things go according to plan, but I can see it being useful
if there's a bug that leads to consumption of a deleted memslot.  Maybe not
"useful" so much as "not misleading".
 
> You mean in kvm_delete_memslot(), am I right?

Yes.

> > It's a single line of code, and there's more than one
> > "shouldn't" in the above.
> 
> If you want, I can both set it and add the comment.  Thanks,

Why bother with the comment?  It'd be wrong in the sense that the as_id is
always valid/accurate, even if npages == 0.