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

From: Paul Durrant <Paul.Durrant@citrix.com>
To: 'Jan Beulich' <JBeulich@suse.com>,
	Petre Ovidiu PIRCALABU <ppircalabu@bitdefender.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>,
	Razvan Cojocaru <rcojocaru@bitdefender.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Andrew Cooper <Andrew.Cooper3@citrix.com>,
	"Tim \(Xen.org\)" <tim@xen.org>,
	George Dunlap <George.Dunlap@citrix.com>,
	JulienGrall <julien.grall@arm.com>,
	Tamas K Lengyel <tamas@tklengyel.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	Alexandru Stefan ISAILA <aisaila@bitdefender.com>,
	Ian Jackson <Ian.Jackson@citrix.com>,
	Roger Pau Monne <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH v2 07/10] vm_event: Add vm_event_ng interface
Date: Wed, 17 Jul 2019 18:42:08 +0000	[thread overview]
Message-ID: <449279104b9d4ac7a7d1b20566097ae7@AMSPEX02CL03.citrite.net> (raw)
In-Reply-To: <20603303-440d-3ac4-8f78-861abaa78d70@suse.com>

> -----Original Message-----
[snip]
> >>> +    }
> >>> +
> >>> +    if ( !vm_event_check(ved) )
> >>> +        return -EINVAL;
> >>> +
> >>> +    if ( frame != 0 || nr_frames != to_channels(ved)->nr_frames )
> >>> +        return -EINVAL;
> >>
> >> Is there a particular reason for this all-or-nothing model?
> >
> > I've added this extra check due to the way acquire_resource interface
> > is designed. In our case, the memory is allocated from
> > XEN_VM_EVENT_ENABLE and the size is known beforehand: the number of
> > pages needed to store (vcpus_count * sizeof vm_event_slot) bytes.
> > However the acquire_resource needs a "nr_frames" parameter which is
> > computed in a similar manner in the libxc wrapper.
> 
> Hmm, maybe I'm not up to date here: Paul, is the general resource
> obtaining/mapping logic indeed meant to be an all-or-nothing thing
> only?
> 

Not really, no. The intention is that any subsection of the resource space may be mapped, so as frame + nr_frames doesn't exceed the size of the space then there should be no reason to return an error.

  Paul

> Jan
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