Re: [PATCH v9 02/18] PCI/P2PDMA: Derive routing from directional ACS controls

[Jason Gunthorpe]

On Fri, Oct 23, 2026 at 01:27:15PM -0500, Bjorn Helgaas wrote:

> > Yes, the ACS combination is selected to avoid ordering issues
> 
> Does a user have any way to know whether the configuration is safe?

No, unfortunately in PCIe so much is really difficult when it comes to
deployment :(

> I'm wondering whether users might use "pci=config_acs=" to configure
> ACS so things appear to work as they prefer but may not provide the
> ordering guarantees drivers depend on.

I imagine that could happen, but on most practical systems the root
complex will blow up if you do asymmetric routing so there is an early
indication.

I know this because I've help debug such configurations at
customers. Some servers helpfully MCE if you do this. Lovely.

> If that's a possibility, do we have any notes in dmesg that we are
> depending on fabric behavior that the OS can't discover and verify?

Maybe, it could be interesting if the P2P logic found asymetric paths
it refuses to permit it.

Jason