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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan


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