Re: [RFC PATCH] iommu/arm-smmu-v3: Allow nested attach for PCI bridges without vDEVICE

[Jason Gunthorpe]

On Mon, Aug 17, 2026 at 11:16:17AM +0000, Pranjal Shrivastava wrote:
> But, regarding PCI P2PDMA, you can still enable it with ACS enabled in
> the upstream kernel *IF* your hardware supports TLP forwarding between
> root ports.
> 
> When ACS is enabled, all TLPs get forwarded to the host's Root Ports
> instead of getting routed directly through the switch.

Generally nobody wants to run like that, the performance is
terrible. The reason these systems have a PCI switch is because of
performance of the P2P path.

Jason