Re: KVM/VFIO passthrough not working when TRIM_UNUSED_KSYMS is enabled

[Paolo Bonzini <pbonzini@redhat.com>]

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
>