> Please read all of my responses. I was explicit about why the check > inside the guest VM produces random results. However, your responses > continue to focus on the guest, which is not the problem.
Fair, my earlier replies were about the guest. Taking the host-side points in turn: 1. Host topology decides P2P, the guest cannot see it, and the patch says "yes" for every pair regardless. The topology is accounted for where the knowledge is: on the host. Which devices land in one VM is decided by the admin, from devices VMware has certified for passthrough in the Compatibility Guide, and on vSphere 8+ by Device Groups, which hands devices sharing a switch or interconnect to a VM as a topology-aware group. Hyper-V has no equivalent grouping; DDA is plain passthrough, each device assigned to the VM explicitly by the admin. The blanket "yes" is not new either. On bare metal the flags = 0 whitelist entries say "yes" for any pair of Xeon Scalable root buses, across sockets, with no knowledge of BIOS P2P settings. And cpu_supports_p2pdma() already bypasses the host bridge check for AMD Zen on CPUID alone, in guests included. This series gives Intel hosts what the tree already does for AMD; if that is unacceptable in principle, it is unacceptable for AMD guests today. 2. Regression: consumers that work today via a system-memory fallback will take the P2P path and break on hosts that cannot route it. A wrong "yes" fails safely: without host-side P2P the request is stopped at the host IOMMU, nothing is corrupted, the same exposure an AMD guest has today. An unconditional "no" is worse: it blocks P2P on every host that does route it, and the guest's answer changes nothing about whether the host routes it. Xeon Scalable routes P2P between any two root ports, which the whitelist already encodes. The only hypervisor-side requirement is mapping the peer's MMIO into the VM's IOMMU domain, and that is what passthrough P2P support means: on ESXi it was an explicit per-VM setting for years and is the default since 9.0; Hyper-V does it for devices assigned to the same VM, as tested on Windows Server 2025. The decision is made on the host; the guest only has to stop vetoing it. 3. The hypervisor must describe it, HMAT is the way, NAK to anything that does not fix the hypervisor first. Agreed HMAT is the right source and should take precedence: consult it first, this check only where no HMAT exists. The disagreement is on "first". Guests on already shipped ESXi and Hyper-V releases will never get HMAT, and those stay in production for years; the cover letter's note on the adoption timeline was written with VMware's input, and Jim can speak to it. Until a hypervisor emits HMAT, this fallback is the only path that works. Thanks, Pavel
