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This would normally happen using ACPI AFAICT >>>>>>> and we >>>>>>> don't have that for the devices behind VMD. >>>>>> >>>>>> In the absence of a mechanism for negotiating ownership, e.g., an >>>>>> ACPI >>>>>> host bridge device for the hierarchy, the OS owns all the PCIe >>>>>> features. >>>>> >>>>> I'm new to this space so I don't know what it means for the OS to >>>>> own the features. In other words, how would the vmd driver figure >>>>> out what features are supported? >>>> >>>> There are three things that go into this: >>>> >>>>     - Does the OS support the feature, e.g., is CONFIG_PCIEAER enabled? >>>> >>>>     - Has the platform granted permission to the OS to use the feature, >>>>       either explicitly via _OSC or implicitly because there's no >>>>       mechanism to negotiate ownership? >>>> >>>>     - Does the device advertise the feature, e.g., does it have an AER >>>>       Capability? >>>> >>>> If all three are true, Linux enables the feature. >>>> >>>> I think vmd has implicit ownership of all features because there is no >>>> ACPI host bridge device for the VMD domain, and (IMO) that means there >>>> is no way to negotiate ownership in that domain. >>>> >>>> So the VMD domain starts with all the native_* bits set, meaning Linux >>>> owns the features.  If the vmd driver doesn't want some feature to be >>>> used, it could clear the native_* bit for it. >>>> >>>> I don't think vmd should unilaterally claim ownership of features by >>>> *setting* native_* bits because that will lead to conflicts with >>>> firmware. >>> >>> This is the crux of the problem IMO. I'm happy to set the native_* bits >>> using some knowledge about what the firmware wants, but we don't have a >>> mechanism to do it AFAICT. I think that's what commit 04b12ef163d1 >>> ("PCI: >>> vmd: Honor ACPI _OSC on PCIe features") was trying to do: use a >>> domain that >>> ACPI had run on and negotiated features and apply them to the vmd >>> domain. >> >> Yes, this is the problem.  We have no information about what firmware >> wants for the VMD domain because there is no ACPI host bridge device. >> >> We have information about what firmware wants for *other* domains. >> 04b12ef163d1 assumes that also applies to the VMD domain, but I don't >> think that's a good idea. >> >>> Using the 3 criteria you described above, could we do this for the >>> hotplug >>> feature for VMD: >>> >>> 1. Use IS_ENABLED(CONFIG_) to check >>> to see >>> if the hotplug feature is enabled >> >> That's already there. >> >>> 2. We know that for VMD we want hotplug enabled so that is the implicit >>> permission >> >> The VMD domain starts with all native_* bits set.  All you have to do >> is avoid *clearing* them. >> >> The problem (IMO) is that 04b12ef163d1 clears bits based on the _OSC >> for some other domain. >> >>> 3. Look at the root ports below VMD and see if hotplug capability is set >> >> This is already there, too. >> >>> If 1 & 3 are true, then we set the native_* bits for hotplug (we can >>> look >>> for surprise hotplug as well in the capability to set the >>> native_shpc_hotplug bit corrrectly) to 1. This feels like it would >>> solve the >>> problem of "what if there is a hotplug issue on the platform" because >>> the >>> user would have disabled hotplug for VMD and the root ports below it >>> would >>> have the capability turned off. >>> >>> In theory we could do this same thing for all the features, but we don't >>> know what the firmware wants for features other than hotplug (because we >>> implicitly know that vmd wants hotplug). I feel like 04b12ef163d1 is >>> a good >>> compromise for the other features, but I hear your issues with it. >>> >>> I'm happy to "do the right thing" for the other features, I just can't >>> figure out what that thing is :) >> >> 04b12ef163d1 was motivated by a flood of Correctable Errors. >> >> Kai-Heng says the errors occur even when booting with >> "pcie_ports=native" and VMD turned off, i.e., when the VMD RCiEP is >> disabled and the NVMe devices appear under plain Root Ports in domain >> 0000.  That suggests that they aren't related to VMD at all. >> >> I think there's a significant chance that those errors are caused by a >> software defect, e.g., ASPM configuration.  There are many similar >> reports of Correctable Errors where "pcie_aspm=off" is a workaround. >> >> If we can nail down the root cause of those Correctable Errors, we may >> be able to fix it and just revert 04b12ef163d1.  That would leave all >> the PCIe features owned and enabled by Linux in the VMD domain.  AER >> would be enabled and not a problem, hotplug would be enabled as you >> need, etc. >> >> There are a zillion reports of these errors and I added comments to >> some to see if anybody can help us get to a root cause. >> > > OK, sounds like the plan for me is to sit tight for now WRT a patch to > fix hotplug. I'll submit a v2 for the documentation. > Any update on fixes for the errors? Paul