Re: [Xen-devel] [PATCH v17 06/11] x86/hvm/ioreq: add a new mappable resource type...

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 05 January 2018 10:29
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: JulienGrall <julien.grall@xxxxxxx>; Andrew Cooper
> <Andrew.Cooper3@xxxxxxxxxx>; George Dunlap
> <George.Dunlap@xxxxxxxxxx>; Ian Jackson <Ian.Jackson@xxxxxxxxxx>; Wei Liu
> <wei.liu2@xxxxxxxxxx>; StefanoStabellini <sstabellini@xxxxxxxxxx>; xen-
> devel@xxxxxxxxxxxxxxxxxxxx; Tim (Xen.org) <tim@xxxxxxx>
> Subject: RE: [Xen-devel] [PATCH v17 06/11] x86/hvm/ioreq: add a new
> mappable resource type...
> 
> >>> On 05.01.18 at 11:16, <Paul.Durrant@xxxxxxxxxx> wrote:
> >> From: Xen-devel [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx] On
> Behalf
> >> Of Paul Durrant
> >> Sent: 03 January 2018 16:48
> >> Ok, thanks. If change back to having the pages owned by the tools domain
> >> then I guess this will all be avoided anyway.
> >
> > I've run into a problem this this, but it may be easily soluable...
> >
> > If I pass back the domid of the resource page owner and that owner is the
> > tools domain, then when the tools domain attempts the mmu_update
> hypercall it
> > fails because it has passed its own domid to mmu_update. The failure is
> > caused by a check in get_pg_owner() which errors own if the passed in
> domid
> > == curr->domain_id but, strangely, not if domid == DOMID_SELF. Any idea
> why
> > this check is there? To me it looks like it should be safe to specify
> > curr->domain_id and have get_pg_owner() simply behave as if
> DOMID_SELF was
> > passed.
> 
> A little while there was some discussion on this general topic (sadly
> I don't recall the context), and iirc it was in particular Andrew (or
> George?) who thought it should be the other way around: If
> DOMID_SELF can be used, the actual domain ID should not be
> accepted (which iirc is currently the case in some places, but not
> in others). But ...
> 
> > The alternative would be to have the acquire_resource hypercall do the
> check
> > and pass back DOMID_SELF is the ioreq server dm domain happens to
> match
> > currd->domain_id, but that seems a bit icky.
> 
> ... this wasn't the plan anyway. Instead we had talked of the
> hypercall returning just a boolean indicator, to distinguish
> self-owned pages from target-domain-owned ones. The
> caller is supposed to know the domain ID of the target domain,
> after all.

Ah, ok. If it's only necessary to distinguish between self-owned and 
target-owned then that will be fine. My current test series just makes the 
domid an IN/OUT parameter and re-writes it if necessary. I'll switch to using a 
flag to avoid the issue as you suggest.

  Paul

> 
> Jan


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