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
