Re: [Qemu-devel] [PATCH v7 0/4] Add Mediated device support

[Neo Jia]

On Wed, Sep 07, 2016 at 07:27:19PM +0100, Daniel P. Berrange wrote:
> On Wed, Sep 07, 2016 at 11:17:39AM -0700, Neo Jia wrote:
> > On Wed, Sep 07, 2016 at 10:44:56AM -0600, Alex Williamson wrote:
> > > On Wed, 7 Sep 2016 21:45:31 +0530
> > > Kirti Wankhede <address@hidden> wrote:
> > > 
> > > > To hot-plug mdev device to a domain in which there is already a mdev
> > > > device assigned, mdev device should be created with same group number as
> > > > the existing devices are and then hot-plug it. If there is no mdev
> > > > device in that domain, then group number should be a unique number.
> > > > 
> > > > This simplifies the mdev grouping and also provide flexibility for
> > > > vendor driver implementation.
> > > 
> > > The 'start' operation for NVIDIA mdev devices allocate peer-to-peer
> > > resources between mdev devices.  Does this not represent some degree of
> > > an isolation hole between those devices?  Will peer-to-peer DMA between
> > > devices honor the guest IOVA when mdev devices are placed into separate
> > > address spaces, such as possible with vIOMMU?
> > 
> > Hi Alex,
> > 
> > In reality, the p2p operation will only work under same translation domain.
> > 
> > As we are discussing the multiple mdev per VM use cases, I think we probably
> > should not just limit it for p2p operation.
> > 
> > So, in general, the NVIDIA vGPU device model's requirement is to 
> > know/register 
> > all mdevs per VM before opening any those mdev devices.
> 
> It concerns me that if we bake this rule into the sysfs interface,
> then it feels like we're making life very hard for future support
> for hotplug / unplug of mdevs to running VMs.

Hi Daniel,

I don't think the grouping will stop anybody from supporting hotplug / unplug at
least from syntax point of view.

> 
> Conversely, if we can solve the hotplug/unplug problem, then we
> potentially would not need this grouping concept.

I think Kirti has also mentioned about hotplug support in her proposal, do you
mind to comment on that thread so I can think if I have missed anything?

Thanks,
Neo

> 
> I'd hate us to do all this complex work to group multiple mdevs per
> VM only to throw it away later when we hotplug support is made to
> work.
> 
> Regards,
> Daniel
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