On Tue, 20 Sep 2016 20:05:01 +0530
Kirti Wankhede <kwankh...@nvidia.com> wrote:

> On 9/20/2016 3:55 AM, Alex Williamson wrote:
> > On Mon, 19 Sep 2016 23:50:56 +0200
> > Paolo Bonzini <pbonz...@redhat.com> wrote:
> >   
> >> On 19/09/2016 23:36, Alex Williamson wrote:  
> >>> On Tue, 20 Sep 2016 02:05:52 +0530
> >>> Kirti Wankhede <kwankh...@nvidia.com> wrote:    
> >>>> 'fb_length':
> >>>>     Read-only file. Mandatory.
> >>>>     Returns <number>{K,M,G}, size of framebuffer.    
> >>>
> >>> This can't be mandatory, it's only relevant to vGPU devices, vGPUs are
> >>> just one user of mediated devices. [...]
> >>>     
> >>>> 'params'
> >>>>     Write-Only file. Optional.
> >>>>     String input. Libvirt would pass the string given in XML file to
> >>>> this file and then create mdev device. Set empty string to clear params.
> >>>> For example, set parameter 'frame_rate_limiter=0' to disable frame rate
> >>>> limiter for performance benchmarking, then create device of type 11. The
> >>>> device created would have that parameter set by vendor driver.    
> >>>
> >>> Might as well just call it "magic", there's absolutely no ability to
> >>> introspect what parameters are allowed or even valid here.  Can all
> >>> parameters be changed dynamically?  Do parameter changes apply to
> >>> existing devices or only future devices?  This is a poorly specified
> >>> interface.  I'd prefer this be done via module options on the vendor
> >>> driver.    
> >>
> >> Or additional files in the mdev's sysfs directory?  
> > 
> > Sure, the vendor driver could certainly support that, it'd be vendor
> > specific of course.
> >  
> Right, it would be vendor specific. But if this is not set through mdev
> device XML, user have to create mdev device with XML first and then
> manually set such parameter. Also these parameters would not retain if
> mdev device is destroyed and created again. That's why if libvirt sets
> these parameters by reading it from XML file, it would be simple for user.
> If we move 'params' file to mdev device, could libvirt do the following
> to create mdev device:
> * Read /sys/../0000\:86\:00.0/11/max_instances. If it is greater than 0,
> then only proceed else fail.
> * Autogenerate UUID
> * Create device:
>     echo "$UUID" > /sys/../0000\:86\:00.0/11/create
> * Set extra params if 'params' field exist in device XML and 'params'
> file exist mdev device directory:
>     echo "frame_rate_limiter=0" > /sys/bus/mdev/devices/$UUID/params

Why does libvirt even need to participate in setting these additional
parameters?  The example here is for benchmarking, which doesn't
express to me a strong need to be automated through libvirt.  For
hostdev devices libvirt supports a managed='no' option that requires
the user to setup the device in advance.  Something similar sounds
reasonable here too.

This is also where I see that a vendor module option could be used,
something like:

modprobe nvidia enable-no-frl-mdev

You say you don't want to apply this to all mdev devices, but maybe it
just makes an additional type-id available "GRID-M60-0B-NOFRL".  Then
you've removed libvirt from the equation, the VM simply specifies this
type-id that incorporates that feature.  Obviously it gets complicated
if there are multiple features to enable, but we've only seen one
example so far of why this parameter is needed.  Thanks,


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