Re: [libvirt] [PATCH v2 4/4] Documentation/vfio-mediated-device.txt: update for aggregation attribute

[Alex Williamson <alex.williamson@redhat.com>]

On Fri, 27 Jul 2018 10:16:58 +0800
Zhenyu Wang <zhenyuw@linux.intel.com> wrote:

> On 2018.07.26 17:46:40 +0200, Cornelia Huck wrote:
> > On Fri, 20 Jul 2018 10:19:28 +0800
> > Zhenyu Wang <zhenyuw@linux.intel.com> wrote:
> >   
> > > Update mdev doc on new aggregration attribute and instances attribute
> > > for mdev.
> > > 
> > > Cc: Kirti Wankhede <kwankhede@nvidia.com>
> > > Cc: Alex Williamson <alex.williamson@redhat.com>
> > > Cc: Kevin Tian <kevin.tian@intel.com>
> > > Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
> > > ---
> > >  Documentation/vfio-mediated-device.txt | 39 ++++++++++++++++++++++----
> > >  1 file changed, 33 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/Documentation/vfio-mediated-device.txt b/Documentation/vfio-mediated-device.txt
> > > index c3f69bcaf96e..9ec9495dcbe7 100644
> > > --- a/Documentation/vfio-mediated-device.txt
> > > +++ b/Documentation/vfio-mediated-device.txt
> > > @@ -211,12 +211,20 @@ Directories and files under the sysfs for Each Physical Device
> > >    |     |   |--- description
> > >    |     |   |--- [devices]
> > >    |     |--- [<type-id>]
> > > -  |          |--- create
> > > -  |          |--- name
> > > -  |          |--- available_instances
> > > -  |          |--- device_api
> > > -  |          |--- description
> > > -  |          |--- [devices]
> > > +  |     |    |--- create
> > > +  |     |    |--- name
> > > +  |     |    |--- available_instances
> > > +  |     |    |--- device_api
> > > +  |     |    |--- description
> > > +  |     |    |--- [devices]
> > > +  |     |--- [<type-id>]
> > > +  |     |    |--- create
> > > +  |     |    |--- name
> > > +  |     |    |--- available_instances
> > > +  |     |    |--- device_api
> > > +  |     |    |--- description
> > > +  |     |    |--- <aggregation>
> > > +  |     |    |--- [devices]
> > >  
> > >  * [mdev_supported_types]
> > >  
> > > @@ -260,6 +268,19 @@ Directories and files under the sysfs for Each Physical Device
> > >    This attribute should show brief features/description of the type. This is
> > >    optional attribute.
> > >  
> > > +* <aggregation>
> > > +
> > > +  The description is to show feature for one instance of the type. <aggregation>  
> > 
> > You are talking about "one instance" here. Can this be different for
> > the same type with different physical devices?
> >  
> 
> I would expect for normal mdev types, driver might expose like x2, x4, x8 types
> which split hw resource equally. But for type with aggregation feature, it can
> set user wanted number of instances. Sorry maybe my use of word was not clear, how
> about "one example of type"?
> 
> > > +  is an optional attributes to show that [<type-id>]'s instances can be
> > > +  aggregated to be assigned for one mdev device. Set number of instances by
> > > +  appending "instances=N" parameter for create. Instances number can't exceed
> > > +  available_instances number. Without "instances=N" parameter will be default
> > > +  one instance to create.  
> > 
> > Could there be a case where available_instances is n, but aggregation
> > is only supported for a value m < n? If yes, should m be discoverable
> > via the "aggregation" attribute?
> >  
> 
> I don't think there could be a case for that. As for aggregation type,
> it represents minimal resource allocated for a singleton, I can't see
> any reason for possible m < n.

Let's think about the interface beyond the immediate needs.  It seems
perfectly reasonable to me to think that a vendor driver might have a
large number of resources available, the ability to aggregate resources
beyond what's practical to enumerate with discrete types, yet have an
upper bound of how many resources can be aggregated for a single
instance.  Thanks,

Alex