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

[Pranjal Shrivastava]

On Mon, Aug 17, 2026 at 08:44:07AM -0300, Jason Gunthorpe wrote:
> 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.

Ack. I was assuming the enablement was the major challenge. For perf we
definitly need to hairpin at the switch. 

> 
> Jason

Thanks,
Praan