Re: [Xen-devel] [PATCH v2 07/10] vm_event: Add vm_event_ng interface

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich <JBeulich@xxxxxxxx>
> Sent: 19 July 2019 13:32
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: 'Petre Ovidiu PIRCALABU' <ppircalabu@xxxxxxxxxxxxxxx>; JulienGrall 
> <julien.grall@xxxxxxx>;
> Alexandru Stefan ISAILA <aisaila@xxxxxxxxxxxxxxx>; Razvan Cojocaru 
> <rcojocaru@xxxxxxxxxxxxxxx>; Andrew
> Cooper <Andrew.Cooper3@xxxxxxxxxx>; George Dunlap <George.Dunlap@xxxxxxxxxx>; 
> Ian Jackson
> <Ian.Jackson@xxxxxxxxxx>; Roger Pau Monne <roger.pau@xxxxxxxxxx>; Stefano 
> Stabellini
> <sstabellini@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx; KonradRzeszutek Wilk
> <konrad.wilk@xxxxxxxxxx>; Tamas K Lengyel <tamas@xxxxxxxxxxxxx>; Tim 
> (Xen.org) <tim@xxxxxxx>; Wei Liu
> <wl@xxxxxxx>
> Subject: Re: [PATCH v2 07/10] vm_event: Add vm_event_ng interface
> 
> On 19.07.2019 14:11, Paul Durrant wrote:
> >> -----Original Message-----
> >> From: Petre Ovidiu PIRCALABU <ppircalabu@xxxxxxxxxxxxxxx>
> >> Sent: 19 July 2019 12:24
> >>
> >> On Fri, 2019-07-19 at 08:26 +0000, Paul Durrant wrote:
> >>>> From: Jan Beulich <JBeulich@xxxxxxxx>
> >>>> Sent: 19 July 2019 09:22
> >>>>
> >>>> On 19.07.2019 09:56, Paul Durrant wrote:
> >>>>> If that page is shared with the ID then
> >>>>> XENMEM_rsrc_acq_caller_owned
> >>>>> should *not* be set. Also, that flag is an 'out' flag... the
> >>>>> caller
> >>>>> doesn't decide who owns the resource.
> >>>>
> >>>> I had implied that it's really MD that's meant here, but maybe I
> >>>> was
> >>>> wrong doing so.
> >>>>
> >>>>> TBH I regret ever introducing the flag; it caused a lot of
> >>>>> problems,
> >>>> which is why it is no longer used.
> >>>>
> >>>> It's a tools only interface - why don't we drop the flag if you now
> >>>> think it was a bad idea to introduce it?
> >>>
> >>> I was indeed thinking I should find enough tuits to do that in the
> >>> near future.
> >>>
> >> Sorry, my mistake. I meant to say it's shared with MD.
> >>
> >> Many thanks for your support,
> >
> > Ok, in that case please share with the ID instead.
> 
> But that's exactly what we want to avoid: If sharing at all, then
> please with the more privileged entity.

Why? We're talking HVM guests only here IIUC so this is equivalent to IOREQ 
server... The pages are target assigned so that foreign mapping works, but 
protected from the guest itself because they are never in the P2M.

> How would MD access the page
> if it's shared with ID (which, aiui, has no business accessing the
> page at all)?
> 

Using foreign mapping in the same way as IOREQ server. Otherwise we are back 
into the ref counting and general accounting hell that target assignment 
avoids. I agree that a better long term solution is probably desirable but I 
don't honestly know what that would look like.

  Paul

> Jan
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