Re: [PATCH v2] PCI: Introduce flag for detached virtual functions

["Oliver O'Halloran" <oohall@gmail.com>]

On Thu, Aug 13, 2020 at 5:21 AM Matthew Rosato <mjrosato@linux.ibm.com> wrote:
>
> s390x has the notion of providing VFs to the kernel in a manner
> where the associated PF is inaccessible other than via firmware.
> These are not treated as typical VFs and access to them is emulated
> by underlying firmware which can still access the PF.  After
> abafbc55 however these detached VFs were no longer able to work
> with vfio-pci as the firmware does not provide emulation of the
> PCI_COMMAND_MEMORY bit.  In this case, let's explicitly recognize
> these detached VFs so that vfio-pci can allow memory access to
> them again.

Hmm, cool. I think we have a similar feature on pseries so that's
probably broken too.

> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
> ---
>  arch/s390/pci/pci.c                |  8 ++++++++
>  drivers/vfio/pci/vfio_pci_config.c | 11 +++++++----
>  include/linux/pci.h                |  1 +
>  3 files changed, 16 insertions(+), 4 deletions(-)
>
> diff --git a/arch/s390/pci/pci.c b/arch/s390/pci/pci.c
> index 3902c9f..04ac76d 100644
> --- a/arch/s390/pci/pci.c
> +++ b/arch/s390/pci/pci.c
> @@ -581,6 +581,14 @@ int pcibios_enable_device(struct pci_dev *pdev, int mask)
>  {
>         struct zpci_dev *zdev = to_zpci(pdev);
>
> +       /*
> +        * If we have a VF on a non-multifunction bus, it must be a VF that is
> +        * detached from its parent PF.  We rely on firmware emulation to
> +        * provide underlying PF details.
> +        */
> +       if (zdev->vfn && !zdev->zbus->multifunction)
> +               pdev->detached_vf = 1;

The enable hook seems like it's a bit too late for this sort of
screwing around with the pci_dev. Anything in the setup path that
looks at ->detached_vf would see it cleared while anything that looks
after the device is enabled will see it set. Can this go into
pcibios_add_device() or a fixup instead?