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

[Gunnar Eggen <geggen54@gmail.com>]

Great! I can confirm that whitelisting the symbols you listed fixes the 
problem.

I hope we get a permanent solution for this eventually, but in the 
meantime we have a workaround.

Thanks so much!


On 05.07.2020 07:44, Paolo Bonzini wrote:
> 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
>>