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From: Paolo Bonzini <pbonzini@redhat.com>
To: Gunnar Eggen <geggen54@gmail.com>,
	alex.williamson@redhat.com, kvm@vger.kernel.org, jeyu@kernel.org
Subject: Re: KVM/VFIO passthrough not working when TRIM_UNUSED_KSYMS is enabled
Date: Sun, 5 Jul 2020 07:44:57 +0200	[thread overview]
Message-ID: <a43675ef-197d-2bd5-9505-200ac439df6c@redhat.com> (raw)
In-Reply-To: <13e90f87-9062-a7e4-99c0-5c6f5c16cad2@gmail.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
> 


  reply	other threads:[~2020-07-05  5:46 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-04 21:03 Gunnar Eggen
2020-07-05  5:44 ` Paolo Bonzini [this message]
2020-07-05  8:58   ` Gunnar Eggen

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