Re: [PATCH] PCI/P2PDMA: start with a whitelist for root complexes

[Bjorn Helgaas <helgaas@kernel.org>]

On Thu, Apr 18, 2019 at 10:58:55AM -0600, Logan Gunthorpe wrote:
> On 2019-04-18 10:33 a.m., Bjorn Helgaas wrote:
> > On Thu, Apr 18, 2019 at 01:58:59PM +0200, Christian König wrote:
> > > A lot of root complexes can still do P2P even when PCI devices
> > > don't share a common upstream bridge.
> > > 
> > > Start adding a whitelist and allow P2P if both participants are
> > > attached to known good root complex.
> > 
> > Is there a plan for addressing this in a generic way that doesn't
> > require an OS modification for every new "known good root complex",
> > e.g., some PCIe or ACPI spec update that allows the OS to discover
> > this?
> 
> I'm aware of work going on in the PCI SIG to address this [1].
> 
> But I expect it's going to be a long time before actual hardware advertises
> this capability to indicate support. So in the interim we either need to not
> use p2pdma on root complexes or create a white list. I'm in favour of the
> white list approach.

I agree we need a whitelist; I just want to make sure we also make
progress on some way to limit the amount of time we need to update it.

> [1] https://lore.kernel.org/linux-pci/20181210115653.0000615a@huawei.com/