On 04/07/20 23:03, Gunnar Eggen wrote: > Hi, > > It's a bit unclear what subsystem is to blame for this problem, so I'm > sending this to both KVM, VFIO and Module support. > > The problem is that trimming unused symbols in the kernel breaks VFIO > passthrough on x86/amd64 at least. If the option TRIM_UNUSED_KSYMS is > enabled you will see the following error when trying to start a VM in > QEmu with any pcie device passed via VFIO: > > qemu-system-x86_64: -device vfio-pci,host=04:00.0: Failed to add group > 25 to KVM VFIO device: Invalid argument > > The error will not stop the VM from launching, but it will break things > in mysterious ways when e.g. installing graphics drivers. > No external modules is involved in this, so I would guess that there is > some dependency that the trimming is missing in some way. > > With the introduction of UNUSED_KSYMS_WHITELIST in the latest kernels, > and some talk about making trimming symbols the default in the future, > it would be great if we could get this fixed or at least identify the > problematic symbols so that they could be whitelisted if needed. They are: - vfio_group_get_external_user - vfio_external_group_match_file - vfio_group_put_external_user - vfio_group_set_kvm - vfio_external_check_extension - vfio_external_user_iommu_id and also (unrelated but breaking other stuff): - mdev_get_iommu_device - mdev_bus_type However, UNUSED_KSYMS_WHITELIST seems the wrong tool for this. We would need to have something that says: "if KVM && VFIO, then include these symbols", for example a macro "IMPORT_SYMBOL" that would be processed by cmd_undef_syms. Paolo > Steps to reproduce: > > 1 - Have a kernel where TRIM_UNUSED_KSYMS is enabled > 2 - Start a VM in QEmu/KVM with a pcie device passed through via vfio-pci > > This is a common issue that keeps popping up on user forums related to > vfio passthrough, so it should be fairly simple to reproduce. > > Let me know if you want more details or perhaps my kernel config or > trimmed system map to test with. > > Best regards, > Gunnar >
