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