Re: [Xen-devel] [PATCH]: xl: pci multi-function passthrough v2

[Gianni Tedesco <gianni.tedesco@xxxxxxxxxx>]

On Tue, 2010-08-10 at 16:25 +0100, Simon Horman wrote:
> On Tue, Aug 10, 2010 at 12:25:46PM +0100, Gianni Tedesco wrote:
> > On Mon, 2010-08-09 at 21:27 +0100, Simon Horman wrote:
> > > On Mon, Aug 09, 2010 at 01:00:39PM +0100, Gianni Tedesco wrote:
> > > > Changes since last time:
> > > >  1. Incorporate Stefanos feedback wrt. coding style, commenting
> > > >     non-obvious code and making single-function a special-case of
> > > >     multi-function
> > > >  2. Also fix the case for passing through a single sub-function and
> > > >     re-mapping it as a single-function virtual device. (ie: pfunc =
> > > >     non-zero, vfunc = zero). Apparently needed for SR-IOV.
> > > >  3. One-liner format change in xl pci-list-assignable to make it
> > > >    print a copy-and-pasteable BDF.
> > > > 8<----------------------------------------
> > > > 
> > > > Implement PCI pass-through for multi-function devices. The supported BDF
> > > > notation is: BB:DD.* - therefore passing-through a subset of functions 
> > > > or
> > > > remapping the function numbers is not supported except for when passing
> > > > through a single function which will be a virtual function 0.
> > > 
> > > Is there any plan to extend this to allow for re-mapping and the like.
> > > When I worked on the original multi-function support (last year)
> > > this seemed to be a requirement of some people.
> > 
> > I am glad you asked
> > 
> > I initially planned to support this but it seemed like a nightmare:
> > 1. The BDF notation practically becomes a regex language ;)
> 
> I don't think its reasonable to say it becomes a regex language.
> But I do agree that it becomes more complex.

Well, for example BB:DD.0=7-7=0 is supposed to reverse the
assignments.... but why? :)

> > 2. For HVM, if a function 0 is not passed through then you don't
> >    generate an SCI interrupt for PCI hotplug.
> 
> Isn't it sufficient to make sure that the guest sees a function 0,
> regardless of what the physical function number is? Or am I missing
> something?

Yes that's all that's required.

> > 3. I couldn't imagine a scenario where this wasn't erroneous thing to do
> 
> I'm not sure that I understand this point.
> I agree that your system should always produce a valid result.
> But I think that there are other configurations that are
> both valid and useful.

Passing various functions in to different VM's and/or re-mapping the
function numbers may produce a totally invalid configuration that isn't
useful (AFAICT). That may be paranoia but I just want to be convinced
that this is actually useful for something.

> > But if someone can convince me that this is a worth-while thing to do
> > (3) then (1) and (2) are just technical problems which can be
> > overcome...
> 
> People convinced me that it was worthwhile, but I'm not those people.

Well, please put them in touch or maybe forward the relevant
discussions? (off-list is OK, if the discussions are private)

Like I say, I am not dead against the idea, I am just loathe to
implement it until I can see what the point of it is.


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