Restore UVM coherence without registering Blackwell BAR1 as P2PDMA

[ducphuc]

Summary

• report UVM coherence from the actual system-memory window again
• keep device_p2p_initialised false for non-coherent Blackwell
• return before static BAR1 can be registered with PCI P2PDMA and have its pagemap operations replaced
• add a debug message exposing static BAR1 size and write-combined state for hardware validation

Static BAR1 is also the GPU peer aperture on this path. Device P2PDMA therefore remains intentionally unavailable on non-coherent Blackwell datacenter GPUs. This matches the current branch effective behavior and does not remove GPU-to-GPU P2P support.

This draft is stacked on the corrected prerequisite branch. Its hardware gate is satisfied, but it should remain draft until aikitoria#33 merges and an upstream one-commit base can be selected.

Validation

• git diff --check fix/610-p2p-safety-prerequisite..HEAD
• complete make modules -j64 build on Linux 7.0.0-28-generic
• test modules signed with the already-enrolled system MOK; the installed signed driver was restored afterward

Stage 1: plain coherence revert

With uvm_debug_prints=1, both GB206 GPUs reached the non-coherent path with static_bar1_write_combined=1, confirming that the existing WC check returns before pci_p2pdma_add_resource(). A prerequisite-only peer-copy run also reproduced the known mailbox assertions fixed by PR aikitoria#31, demonstrating why the final copy validation must include that PR.

Stage 2: final guard plus PR aikitoria#31 integration

• the defensive skip diagnostic fired for both non-coherent GB206 GPUs
• simpleP2P passed bidirectional data verification at about 13.09 GB/s
• p2pBandwidthLatencyTest reported 14.09 GB/s per direction and about 27.8 GB/s aggregate bidirectional
• nvidia-smi topo -p2p: read, write, and PCIe are OK; atomics are NS, not DR
• no Xids, assertions, BAR1 failures, IOMMU faults, or P2PDMA conflicts appeared in the final test log

[ducphuc]

Closing as superseded by production/runtime-bar1-coverage. No further review is needed; the head branch is retained temporarily for historical references.