Bug#1015871: Enabling PCI_P2PDMA for distro kernels?

[Bjorn Helgaas <helgaas@kernel.org>]

On Wed, Oct 25, 2023 at 07:11:26PM +0200, Lukas Wunner wrote:
> On Wed, Oct 25, 2023 at 10:30:07AM -0600, Logan Gunthorpe wrote:
> > In addition to the above, P2PDMA transfers are only allowed by the
> > kernel for traffic that flows through certain host bridges that are
> > known to work. For AMD, all modern CPUs are on this list, but for Intel,
> > the list is very patchy.
> 
> This has recently been brought up internally at Intel and nobody could
> understand why there's a whitelist in the first place.  A long-time PCI
> architect told me that Intel silicon validation has been testing P2PDMA
> at least since the Lindenhurst days, i.e. since 2005.
> 
> What's the reason for the whitelist?  Was there Intel hardware which
> didn't support it or turned out to be broken?
> 
> I imagine (but am not certain) that the feature might only be enabled
> for server SKUs, is that the reason?

No, the reason is that the PCIe spec doesn't require routing of
peer-to-peer transactions between Root Ports:
https://git.kernel.org/linus/0f97da831026

I think there was a little discussion about adding a firmware
interface to advertise this capability, but I guess nobody cared
enough to advance it.

Bjorn