Re: [Xen-devel] [v1 1/2] xen/p2m: Create identity mappings for PFNsbeyound E820 and PCI BARs

From: Gordan Bobic
Date: Tue Oct 29 2013 - 12:09:06 EST

On Tue, 29 Oct 2013 11:11:29 -0400, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> wrote:
On Tue, Oct 29, 2013 at 02:55:13PM +0000, Jan Beulich wrote:
>>> On 29.10.13 at 15:45, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> wrote:
> On Tue, Oct 29, 2013 at 08:23:30AM +0000, Jan Beulich wrote:
>> >>> On 28.10.13 at 17:58, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> wrote:
>> > On Fri, Oct 25, 2013 at 04:08:19PM -0600, Bjorn Helgaas wrote:
>> >> If you can look at PCI host bridge apertures instead of BARs, that
>> >> would solve both problems. Reassigning those apertures is
>> >> theoretically possible but is not even a gleam in our eyes yet.
>> >
>> > <nods> I think I have to have both (BARs and host bridge apertures) as when
>> > we do PCI passthrough to a guest - we might do it without a bridge.
>> Why? Aren't the host bridge ranges necessarily a superset of the
>> individual devices' BARs?
> Yes. But when you pass in a PCI device to a PV guest you don't pass in the
> bridge. Just the PCI device itself.

Right you are. Which means that basing the whole logic on the
PCI device BARs is likely wrong anyway, not just because it
doesn't account for other MMIO ranges.

Right, but that is OK. When you pass in a PCI device to a PV guest you
only care about that specific device driver being able to access its BARs.

Maybe I'm reading this wrong, but my understanding is that you cannot
pass PCI bridges to domU anyway (in HVM - I tried). Is that not the case?
I'm particularly interested in this for two reasons:

1) Some GPUs (Nvidia?) use bus resets to reset the GPU

2) Multi-GPU cards (e.g. GTX690/Grid K2) come with a PCIe bridge
of their own. I am successfully passing a modified GTX680 (as either
a Grid K2 or Quadro K5000) to a domU, but have completely failed to
get a modified GTX690 (Grid K2, exact same GPU as the GTX680) to work
with passthru. The only theory I have is that the extra PCIe bridge
is the problem (possibly a compound problem that only manigests when
there is a PLX (as per GTX690) PCIe bridge behind a NF200 PCIe bridge,
behind an Intel PCIe bridge.

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