Re: KVM call minutes 2013-01-29 - Port I/O

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Thu, 2013-01-31 at 12:49 +0200, Michael S. Tsirkin wrote:

> OK but this appears behind a bridge.  So the bridge configuration tells
> the root complex where to send accesses to the VGA.

Sort-of, again the root complex isn't "sending" anything targeted here.
PCIe is point to point and any device is behind a bridge, real or
virtual.

> But qemu currently puts devices directly on root bus.

Sure, because qemu doesn't specifically model PCIe but something "else"

> And as far as I can tell when we present devices directly on bus 0, we
> pretend these are integrated in the root complex.

Right, it's a bit gross.

>  The spec seems to
> say explicitly that root complex integrated devices should not use legacy
> addresses or support hotplug. So I would be surprised if such one
> appears in real world.

Sure but that doesn't change the fact that there's no point in treating
things differently between PCI and PCIe for the sake of address range
decoding. The high level model remains the same.

> Luckily guests do not seem to be worried as long as we use ACPI.

Right, it all just looks like PCI to the guest anyway and is mostly
treated as such for the sake of routing and decoding (until you turn on
ARI but that's a different can of worms).

> > BTW, I've been working on vfio-pci support of VGA assignment which makes
> > use of the VGA arbiter in the host to manipulate the VGA Enable control
> > register, allowing us to select which device to access.  The qemu side
> > is simply registering memory regions for the VGA areas and expecting to
> > be used with -vga none, but I'll adopt whatever strategy we choose for
> > hard coded address range support.  Current base patches at the links
> > below.  Thanks,
> > 
> > Alex
> > 
> > https://github.com/awilliam/qemu-vfio/commit/ea2befa59010a429dcf13c10dbccdf8b64e82fbd
> > https://github.com/awilliam/linux-vfio/commit/bae182d929229cbf1eaeb01e5fad4f77f81a4c61