Re: [Xen-devel] [PATCH v2 REPOST 12/12] x86/hvm/ioreq: add a new mappable resource type...

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: George Dunlap
> Sent: 29 August 2017 15:38
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>; Wei Liu
> <wei.liu2@xxxxxxxxxx>; Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Tim
> (Xen.org) <tim@xxxxxxx>; Jan Beulich <jbeulich@xxxxxxxx>; Ian Jackson
> <Ian.Jackson@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx; Roger Pau
> Monne <roger.pau@xxxxxxxxxx>
> Subject: Re: [Xen-devel] [PATCH v2 REPOST 12/12] x86/hvm/ioreq: add a
> new mappable resource type...
> 
> On Tue, Aug 29, 2017 at 3:31 PM, Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> wrote:
> >> -----Original Message-----
> > [snip]
> >> >> I'm not terribly happy with allocating out-of-band pages either.  One
> >> >> of the advantages of the way things are done now (with the page
> >> >> allocated to the guest VM) is that it is more resilient to unexpected
> >> >> events:  If the domain dies before the emulator is done, you have a
> >> >> "zombie" domain until the process exits.  But once the process exits
> >> >> for any reason -- whether crashing or whatever -- the ref is freed and
> >> >> the domain can finish dying.
> >> >>
> >> >> What happens in this case if the dm process in dom0 is killed /
> >> >> segfaults before it can unmap the page?  Will the page be properly
> >> >> freed, or will it just leak?
> >> >
> >> > The page is referenced by the ioreq server in the target domain, so it
> will
> >> be freed when the target domain is destroyed.
> >>
> >> I don't understand how you're using the terms... I would have
> >> interpreted 'target domain' to me means the guest VM to which
> emulated
> >> devices are being provided, and 'ioreq server' means the process
> >> (perhaps in dom0, perhaps in a stubdomain) which is providing the
> >> emulated devices.
> >>
> >> Did you mean that it's referenced by the ioreq_server struct in the
> >> target domain, and so a put_page() will happen when the guest is
> >> destroyed?
> >
> > Terminology issues :-) By 'ioreq server' I mean the infrastructure in Xen,
> centred around struct ioreq_server. I refer to the dom0 process/stub
> domain/xengt module/whatever as the 'emulator'.
> > So, yes, the fact that the page is referenced in the ioreq server of the
> target domain means that a put_page() will happen when that domain is
> destroyed.
> 
> OK; in that case:
> 
> Acked-by: George Dunlap <george.dunlap@xxxxxxxxxx>
> 

Cool, thanks.

> I think that's the only other Ack you need from me.  Thanks for doing this
> work.
> 

No probs. Cheers,

  Paul

>  -George
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