Re: [Xen-devel] [v5][PATCH 0/5] xen: add Intel IGD passthrough support

[Paolo Bonzini <pbonzini@xxxxxxxxxx>]

Il 26/06/2014 13:26, Michael S. Tsirkin ha scritto:
> On Thu, Jun 26, 2014 at 12:03:58PM +0200, Paolo Bonzini wrote:
> > Il 26/06/2014 11:18, Chen, Tiejun ha scritto:
> > > > - offsets 0x0000..0x0fff map to configuration space of the host MCH
> > > Are you saying the config space in the video device?
> > No, I am saying in a new BAR, or at some magic offset of an existing MMIO
> > BAR.
> >
> > Paolo
> I think v5 goes a bit overboard and overrides too many registers,
> driver likely needs much less.
>
> But IMHO retro-fitting this into PIIX is where the problem is.
> We could invent 1000 ways to do this, all of varying levels of
> ugliness.
>
> But why don't we start by more or less cleanly emulating what the driver
> needs?  If we start with Q35 - it shouldn't be hard to tweak it's MCH to
> add the necessary config space, right?
>
> I know xen doesn't support q35 now but it's opensource :)
>
> I think that's a good step 1. We can do PV hacks on top if we
> decide they aren't too painful.
I don't think it's a PV hack.  It should be this way in real hardware 
too (not exactly _this_ hack, but at least no improper relationship 
among IGD/MCH/PCH).
Once you add nested virtualization and nested device assignment to the 
mix, hacking subsystem IDs simply does not scale.
Paolo

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