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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Petre Ovidiu PIRCALABU <ppircalabu@xxxxxxxxxxxxxxx>
> Sent: 19 July 2019 18:40
> To: Jan Beulich <JBeulich@xxxxxxxx>; Paul Durrant <Paul.Durrant@xxxxxxxxxx>; 
> Andrew Cooper
> <Andrew.Cooper3@xxxxxxxxxx>
> Cc: JulienGrall <julien.grall@xxxxxxx>; Alexandru Stefan ISAILA 
> <aisaila@xxxxxxxxxxxxxxx>; Razvan
> Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>; 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 Fri, 2019-07-19 at 12:59 +0000, Jan Beulich wrote:
> > On 19.07.2019 14:37, Paul Durrant wrote:
> > > > From: Jan Beulich <JBeulich@xxxxxxxx>
> > > > Sent: 19 July 2019 13:32
> > > >
> > > > On 19.07.2019 14:11, Paul Durrant wrote:
> > > > > > -----Original Message-----
> > > > > > From: Petre Ovidiu PIRCALABU <ppircalabu@xxxxxxxxxxxxxxx>
> > > > > > Sent: 19 July 2019 12:24
> > > > > >
> > > > > > 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...
> >
> > Not sure: The main vm_event.c files live in common/ and arch/x86/
> > respectively, so I thought at least architecturally VM events were
> > possible for PV as well. If it's indeed HVM-only, then following
> > the IOREQ server model in its entirety would of course be fine.
> >
> > Jan
> 
> In one of the previous version of the patchset there was a suggestion
> to implement the new vm_event transport using IOREQ, but it was dropped
> .
> 
> https://lists.xenproject.org/archives/html/xen-devel/2019-04/msg00173.html
> 
> Also, unless there isn't a proper way allocate the necessary pages, I
> wouldn't introduce a HVM-only limitation because, other than the HVM
> param used to keep track of the ring pfn, the vm_event mechanism is
> quite generic.

Well, with resource mapping you wouldn't need the HVM param, presumably. IMO it 
would be best to make this HVM only in the first instance, to avoid blocking 
progress. This case does highlight the need for a solution to the issue of 
resource accounting for PV guests. My suggestion is a page type that can be 
assigned to a guest but cannot be mapped to that guest... something akin to a 
page table perhaps?

  Paul

> 
> Many thanks for your support,
> Petre

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