On Apr 29, 2026, at 2:38 PM, Jason Gunthorpe [off-list ref] wrote:

!-------------------------------------------------------------------|
 This Message Is From an External Sender
 This message came from outside your organization.
|-------------------------------------------------------------------!

On Tue, Apr 28, 2026 at 08:04:14PM +0200, Max Boone via B4 Relay wrote:

quoted

From: Max Boone <redacted>

When MLX5 cards are passed through to a VM, disabling SR-IOV by
setting the sriov_numvfs to 0 will render the machine unstable.

What? How does that happen?

Unstable is maybe a bit confusing phrasing on my part, “locks up”
might be a better description?

In short:
- Enable by setting sriov_numvfs to positive
- vfio-pci passthrough to QEMU (or other process)
- Disable by setting sriov_numvfs to zero
- QEMU processes freeze, shell that was writing to sysfs freezes
- SIGKILL doesn’t seem to have much effect, shutdown never completes

Python script to reproduce without QEMU:
- https://github.com/akamaxb/repro-vfio-sriov-removal/blob/main/vfio-sriov-bind.py

Does:
  1. Require sriov_numvfs == 0 on the PF (report any existing users and exit if not)
  2. Add one SR-IOV VF
  3. Bind the VF to vfio-pci via driver_override + drivers_probe
  4. Open VFIO container + group, get device fd
  5. Create a KVM VM (registers an MMU notifier — required to trigger the race)
  6. Hold and wait for user input

To trigger the bug while the script is waiting, in another terminal:
    echo 0 > /sys/bus/pci/devices/<pf_device>/sriov_numvfs

On the vfio-pci end of it all, it prints these two lines to dmesg before it hangs:
- https://elixir.bootlin.com/linux/v7.0.1/source/drivers/vfio/pci/vfio_pci_core.c#L1826
- https://elixir.bootlin.com/linux/v7.0.1/source/drivers/vfio/vfio_main.c#L421

quoted

-void mlx5_sriov_disable(struct pci_dev *pdev, bool num_vf_change)
+int mlx5_sriov_disable(struct pci_dev *pdev, bool num_vf_change)
{
struct mlx5_core_dev *dev  = pci_get_drvdata(pdev);
struct devlink *devlink = priv_to_devlink(dev);
int num_vfs = pci_num_vf(dev->pdev);

+ if (pci_vfs_assigned(dev->pdev)) {
+ mlx5_core_warn(dev, "can't disable sriov, VFs are assigned\n");
+ return -EPERM;
+ }

*barf* WTF did this come from?

Hahaha, take your pick:
- https://elixir.bootlin.com/linux/v7.0.1/C/ident/pci_vfs_assigned

I followed the sysfs sriov_numvfs op for a couple drivers and saw
that ixgbe (and others) had it plumbed in, so presumed (sorry)
that this would fix it / was an obvious omission if the rest is doing
it. My bad for cargo culting an artifact from Xen.

Grep says only Xen makes this true, so this is all working around some
Xen brokenness in their "assignment" ?

Yeap, I see, looks like it.

If people care about Xen pci_is_dev_assigned() should be be purged and
pciback should be fixed to not "make the machine unstable" when it is
removed during a VF teardown.

Or at the very least this nasty Xen intrustion should be placed in the
PCI core code and removed from the drivers.

Also, no, you can't fail mlx5_sriov_disable() it is called during
driver remove and cannot fail in that flow.

Check. I can do some further digging and build a kernel with lockdep
to try and find what it is hanging on specifically. Unless something pops
to mind?

Jason