Bjorn, Logan, Leon's position is clear: "NAK to anything that does not fix the hypervisor first". No concrete failure was described, and no change in this series can address it. That leaves the decision with you. Two facts: AMD guests get P2PDMA today and Intel guests do not, and HMAT does not exist in Linux yet.
Thanks, Pavel > -----Original Message----- > From: Leon Romanovsky <[email protected]> > Sent: Friday, October 16, 2026 8:32 AM > To: Popov, Pavel E <[email protected]> > Cc: Bjorn Helgaas <[email protected]>; Logan Gunthorpe > <[email protected]>; Jim Chow <[email protected]>; Bjorn Helgaas > <[email protected]>; Radu Rugina <[email protected]>; Alexey > Makhalov <[email protected]>; Wei Liu <[email protected]>; > Michael Kelley <[email protected]>; Lukas Wunner <[email protected]>; > Rafael J. Wysocki <[email protected]>; Nathan Ciobanu > <[email protected]>; [email protected]; > [email protected]; [email protected]; linux- > [email protected]; [email protected] > Subject: Re: [PATCH 0/2] PCI/P2PDMA: Allow P2PDMA in VMware and Hyper-V > guests on Intel hosts > > On Fri, Oct 16, 2026 at 02:55:07PM +0000, Popov, Pavel E wrote: > > Bjorn, Logan, > > > > Could you consider taking this series despite the NAK, for two reasons: > > > > - Parity: Linux gives AMD guests preferential treatment > > This is misleading. You did not propose adding Intel processors to a > whitelist, for > example in `cpu_supports_p2pdma()`. Instead, you explicitly added a check for > whether the system is running in a VM. > > > - No alternative: the HMAT solution doesn't exist in Linux yet > > This is not how open-source development works. Let's work together to find a > solution that works for everyone. > > Thanks > > > > > Thanks, > > Pavel > > > > > -----Original Message----- > > > From: Leon Romanovsky <[email protected]> > > > Sent: Tuesday, October 13, 2026 5:20 AM > > > To: Popov, Pavel E <[email protected]> > > > Cc: Bjorn Helgaas <[email protected]>; Jim Chow > > > <[email protected]>; Bjorn Helgaas <[email protected]>; Logan > > > Gunthorpe <[email protected]>; Radu Rugina > > > <[email protected]>; Alexey Makhalov > > > <[email protected]>; Wei Liu <[email protected]>; > > > Michael Kelley <[email protected]>; Lukas Wunner > > > <[email protected]>; Nathan Ciobanu > > > <[email protected]>; bcm-kernel-feedback- > > > [email protected]; [email protected]; > > > [email protected]; [email protected]; > > > [email protected] > > > Subject: Re: [PATCH 0/2] PCI/P2PDMA: Allow P2PDMA in VMware and > > > Hyper-V guests on Intel hosts > > > > > > On Mon, Oct 12, 2026 at 08:01:40PM +0000, Popov, Pavel E wrote: > > > > > 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. > > > > > > If you had simply added Intel CPUs to the list of supported P2P > > > devices, I might have been willing to accept that. However, you added > > > these > two lines: > > > > > > + (hypervisor_is_type(X86_HYPER_VMWARE) || > > > + hypervisor_is_type(X86_HYPER_MS_HYPERV))) > > > > > > This makes your case quite different from AMD's. Their check was for > > > hypervisor, which leaked to VMs. And, yes, AMD does not work reliably in > guest VMs either. > > > For our internal testing, we have carefully curated hypervisor > > > configurations and test VMs, along with custom patches, to test P2P > > > reliably. > > > > > > > > > > > 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. > > > > > > This is VMware's problem, not the upstream kernel community's. If > > > VMware considers P2P important and its customers agree, VMware can > upgrade. > > > > > > It is no different from any other kernel feature. > > > > > > Thanks > > > > > > > > > > > Thanks, > > > > Pavel > > > > > >
