[Intel-gfx] [RFC PATCH 0/8] Add host i915 support for vGPU

[Tian, Kevin]

> From: Daniel Vetter [mailto:daniel.vetter at ffwll.ch] On Behalf Of Daniel Vetter
> Sent: Thursday, October 23, 2014 4:56 PM
> 
> On Thu, Oct 23, 2014 at 11:13:51AM +0800, Jike Song wrote:
> > On 10/22/2014 05:48 PM, Daniel Vetter wrote:
> > >So on a very high level I don't understand this design. For the guest side
> > >it's completely clear that we need a bunch of hooks over the driver to
> > >make paravirtualization work.
> > >
> > >But on the host side I expect the driver to be in full control of the
> > >hardware. I haven't really seen the other side but it looks like with vgt
> > >we actually have two drivers fighting over the hardware, which requires
> > >the various hooks to avoid disaster. The drm subsystem is littered with
> > >such attempts, and they all didn't end up in a pretty way.
> > >
> > >So way can't we have the vgt support for guests sit on top of i915, using
> > >the i915 functions to set up pagetables, contexts and reserve gtt areas
> > >for the guests? Then we'd have just one driver in control of the hardware,
> > >and vgt on the host side would just look like a really crazy interace
> > >layer between virtual hosts and the low-level driver, similar to who the
> > >execbuf ioctl is a really crazy interface between userspace and the
> > >low-level driver.
> >
> >  Yes we can do this, but that also means lots of pollution to existing i915
> > codes, only for virtualization purpose. Currently vgt has pretty abstractions
> > for both host i915 and guest graphics drivers, mixing vgt and host i915
> means
> > breaking that.  I believe a vgt-integrated repository will be better to look
> > at :)
> 
> The problem is that we need to have this integration anyway. Using the
> current design it will be hidden behind some thin abstraction, but it will
> be as invasive and fragile as if the interactions is made explicit.
> 
> Stuf like driver load/unload, suspend/resume, runtime pm and gpu reset are
> already supre-fragile as-is. Every time we change something in there, a
> bunch of related things fall apart. With vgt we'll have even more
> complexity in there, and I really think we need to make that complexity
> explicit. Otherwise we'll always break vgt support for host systems by
> accident when working on upstream. So in my experience being explicit with
> these depencies massively reduces maintaince headaches longterm.
> 
> Of course for starting up the vgt effort it's better to have as much
> separation as possible. But the balance between development and
> maintainability do change quite a bit when merging code upstream.

Yep. Just synced with Jike. Actually this is the way we'd also prefer to, 
which makes vgt a more integrated feature in i915 driver. We'll think 
about all the technical detail along this direction, and report back any 
design issue to discuss in advance. In the meantime, we'll try to provide 
a complete repo including vgt component, in next version, so you can 
see clearly the interactions between them. Likely it will take more time 
than original approach, but it's worthy of doing so... :-)

> 
> Another benefit of the inverted design with vgt sitting on top of normal
> i915 for host support is better automated testing. At least with kvm we
> could then simply create a kvm guest without any special boot parameters
> and run some really basic testcases in the guest to make sure vgt doesn't
> get broken. That would fit rather nicely into i-g-t. Of course with xengt
> we unfortunately can't do that, since we'd need to boot with the
> hypervisor.

yes, that'd be a nice step. kvm support is important in our plan, though
it's now behind Xen side regarding to quality.

> 
> But I think without such smoketesting in the automated upstream test suite
> we'll break vgt support constantly in upstream. So I think we really need
> this too, at least long term.
> 
> I hope that explains a bit where I'm coming from. Note that this is just
> about the host side, imo the guest side can be merged as soon as detailed
> review has been completed.


Thanks
Kevin