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Tue, 9 Jul 2024 22:15:15 +0000 Date: Tue, 9 Jul 2024 15:15:13 -0700 From: Dan Williams To: James Bottomley , Dan Williams , CC: , , , Subject: Re: [MAINTAINERS SUMMIT] Device Passthrough Considered Harmful? 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Please consider it for a Maintainers Summit discussion. > > I'm with Greg on this ... can you point to some of the contrary > positions? This thread has that discussion: http://lore.kernel.org/0-v1-9912f1a11620+2a-fwctl_jgg@nvidia.com I do not want to speak for others on the saliency of their points, all I can say is that the contrary positions have so far not moved me to drop consideration of fwctl for CXL. Where CXL has a Command Effects Log that is a reasonable protocol for making decisions about opaque command codes, and that CXL already has a few years of experience with the commands that *do* need a Linux-command wrapper. Some open questions from that thread are: what does it mean for the fate of a proposal if one subsystem Acks the ABI and another Naks it for a device that crosses subsystem functionality? Would a cynical hardware response just lead to plumbing an NVME admin queue, or CXL mailbox to get device-specific commands past another subsystem's objection? My reconsideration of the "debug-build only" policy for CXL device-specific commands was influenced by a conversation with a distro developer where they asserted, paraphrasing: "at what point is a device vendor incentivized to ship an out-of-tree module just to restore their passthrough functionality?. At that point upstream has lost out on collaboration and distro kernel ABI has gained another out-of-tree consumer." So the tension is healthy, but it has diminishing returns past a certain point.