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

["Michael S. Tsirkin" <mst@xxxxxxxxxx>]

On Wed, Jul 02, 2014 at 06:29:23PM +0200, Paolo Bonzini wrote:
> Il 02/07/2014 17:27, Michael S. Tsirkin ha scritto:
> > At some level, maybe Paolo is right.  Ignore existing drivers and ask
> > intel developers to update their drivers to do something sane on
> > hypervisors, even if they do ugly things on real hardware.
> > 
> > A simple proposal since what I wrote earlier though apparently wasn't
> > very clear:
> > 
> >   Detect Xen subsystem vendor id on vga card.
> >   If there, avoid poking at chipset. Instead
> >     - use subsystem device # for card type
> 
> You mean for PCH type (aka PCH device id).
> 
> >     - use second half of BAR0 of device
> >     - instead of access to pci host
> > 
> > hypervisors will simply take BAR0 and double it in size,
> > make second part map to what would be the pci host.
> 
> Nice.  Detecting the backdoor via the subsystem vendor id
> is clever.
> 
> I'm not sure if it's possible to just double the size of BAR0 
> or not,

Why won't it be?
You just make one bit that is RO in hw RW in QEMU.

> but my laptop has:
> 
>       Region 0: Memory at d0000000 (64-bit, non-prefetchable) [size=4M]
>       Region 2: Memory at c0000000 (64-bit, prefetchable) [size=256M]
>       Region 4: I/O ports at 5000 [size=64]
> 
> and I hope we can reserve a few KB for hypervisors within those
> 4M, or 8 bytes for an address/data pair (like cf8/cfc) within BAR4's
> 64 bytes (or grow BAR4 to 128 bytes, or something like that).

Wasting IO isn't a good idea usually.

> Xen can still add the hacky machine type if they want for existing 
> hosts, but this would be a nice way forward.
> 
> Paolo

We need to get agreement from driver writers though,
specifically windows guys.

-- 
MST

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