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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 03.01.18 at 17:06, <Paul.Durrant@xxxxxxxxxx> wrote:
>>  -----Original Message-----
>> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: 03 January 2018 15:48
>> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
>> Cc: JulienGrall <julien.grall@xxxxxxx>; Andrew Cooper
>> <Andrew.Cooper3@xxxxxxxxxx>; Wei Liu <wei.liu2@xxxxxxxxxx>; George
>> Dunlap <George.Dunlap@xxxxxxxxxx>; Ian Jackson <Ian.Jackson@xxxxxxxxxx>;
>> Stefano Stabellini <sstabellini@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx;
>> Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>; Tim (Xen.org)
>> <tim@xxxxxxx>
>> Subject: Re: [PATCH v17 06/11] x86/hvm/ioreq: add a new mappable
>> resource type...
>> 
>> >>> On 03.01.18 at 13:19, <paul.durrant@xxxxxxxxxx> wrote:
>> > +static void hvm_free_ioreq_mfn(struct hvm_ioreq_server *s, bool buf)
>> > +{
>> > +    struct domain *d = s->domain;
>> > +    struct hvm_ioreq_page *iorp = buf ? &s->bufioreq : &s->ioreq;
>> > +
>> > +    if ( !iorp->page )
>> > +        return;
>> > +
>> > +    page_list_add_tail(iorp->page, &d-
>> >arch.hvm_domain.ioreq_server.pages);
>> 
>> Afaict s->domain is the guest, not the domain containing the
>> emulator. Hence this new model of freeing the pages is safe only
>> when the emulator domain is dead by the time the guest is being
>> cleaned up.
> 
> From the investigations done w.r.t. the grant table pages I don't think this 
> is the case. The emulating domain will have references on the pages and this 
> keeps the target domain in existence, only completing domain destruction when 
> the references are finally dropped. I've tested this by leaving an emulator 
> running whilst I 'xl destroy' the domain; the domain remains as a zombie 
> until emulator terminates.

Oh, right, I forgot about that aspect.

>> What is additionally confusing me is the page ownership: Wasn't
>> the (original) intention to make the pages owned by the emulator
>> domain rather than the guest? I seem to recall you referring to
>> restrictions in do_mmu_update(), but a domain should always be
>> able to map pages it owns, shouldn't it?
> 
> I'm sure we had this discussion before. I am trying to make resource mapping 
> as uniform as possible so, like the grant table pages, the ioreq server pages 
> are assigned to the target domain. Otherwise the domain trying to map 
> resources has know which actual domain they are assigned to, rather than the 
> domain they relate to... which is pretty ugly.

Didn't I suggest a slight change to the interface to actually make
this not as ugly?

>> Furthermore you continue to use Xen heap pages rather than
>> domain heap ones.
> 
> Yes, this seems reasonable since Xen will always need mappings of the pages 
> and the aforementioned reference counting only works for Xen heap pages AIUI.

share_xen_page_with_guest() makes any page a Xen heap one.
See vmx_alloc_vlapic_mapping() for an example.

Jan


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