Re: [Xen-devel] [RFC PATCH 4/6] vm_event: Use slotted channels for sync requests.

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Petre Ovidiu PIRCALABU [mailto:ppircalabu@xxxxxxxxxxxxxxx]
> Sent: 10 January 2019 15:31
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>; Wei Liu
> <wei.liu2@xxxxxxxxxx>; Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>; Konrad
> Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>; George Dunlap
> <George.Dunlap@xxxxxxxxxx>; Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Ian
> Jackson <Ian.Jackson@xxxxxxxxxx>; Tim (Xen.org) <tim@xxxxxxx>; Julien
> Grall <julien.grall@xxxxxxx>; Tamas K Lengyel <tamas@xxxxxxxxxxxxx>; Jan
> Beulich <jbeulich@xxxxxxxx>; Roger Pau Monne <roger.pau@xxxxxxxxxx>
> Subject: Re: [Xen-devel] [RFC PATCH 4/6] vm_event: Use slotted channels
> for sync requests.
> 
> On Thu, 2018-12-20 at 12:05 +0000, Paul Durrant wrote:
> > > -----Original Message-----
> > >
> > > The memory for the asynchronous ring and the synchronous channels
> > > will
> > > be allocated from domheap and mapped to the controlling domain
> > > using the
> > > foreignmemory_map_resource interface. Unlike the current
> > > implementation,
> > > the allocated pages are not part of the target DomU, so they will
> > > not be
> > > reclaimed when the vm_event domain is disabled.
> >
> > Why re-invent the wheel here? The ioreq infrastructure already does
> > pretty much everything you need AFAICT.
> >
> >   Paul
> >
> 
> Hi Paul,
> 
> I'm still struggling to understand how the vm_event subsystem could be
> integrated with an IOREQ server.
> 
> An IOREQ server shares with the emulator 2 pages, one for ioreqs and
> one for buffered_ioreqs. For vm_event we need to share also one or more
> pages for the async ring and a few pages for the slotted synchronous
> vm_events.
> So, to my understanding, your idea to use the ioreq infrastructure for
> vm_events is basically to replace the custom signalling (event channels
> + ring / custom states) with ioreqs. Since the
> vm_event_request/response structures are larger than 8 bytes, the
> "data_is_ptr" flag should be used in conjunction with the addresses
> (indexes) from the shared vm_event buffers.
> 
> Is this the mechanism you had in mind?
> 

Yes, that's roughly what I hoped might be possible. If that is too cumbersome 
though then it should at least be feasible to mimic the ioreq code's page 
allocation functions and code up vm_event buffers as another type of mappable 
resource.

  Paul

> Many thanks,
> Petre

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