Subject: Re: [PATCH V1 mlx5-next 12/13] vfio/pci: Add infrastructure to let vfio_pci_core drivers trap device RESET

From: Jason Gunthorpe <[email-masked]>
Date: Mon, 18 Oct 2021 09:02:34 -0300

On Sun, Oct 17, 2021 at 05:29:39PM +0300, Yishai Hadas wrote:
> On 10/16/2021 12:12 AM, Alex Williamson wrote:
> > On Fri, 15 Oct 2021 17:03:28 -0300
> > Jason Gunthorpe <[email-masked]> wrote:
> > 
> > > On Fri, Oct 15, 2021 at 01:52:37PM -0600, Alex Williamson wrote:
> > > > On Wed, 13 Oct 2021 12:47:06 +0300
> > > > Yishai Hadas <[email-masked]> wrote:
> > > > > Add infrastructure to let vfio_pci_core drivers trap device RESET.
> > > > > 
> > > > > The motivation for this is to let the underlay driver be aware that
> > > > > reset was done and set its internal state accordingly.
> > > > I think the intention of the uAPI here is that the migration error
> > > > state is exited specifically via the reset ioctl.  Maybe that should be
> > > > made more clear, but variant drivers can already wrap the core ioctl
> > > > for the purpose of determining that mechanism of reset has occurred.
> > > It is not just recovering the error state.
> > > 
> > > Any transition to reset changes the firmware state. Eg if userspace
> > > uses one of the other emulation paths to trigger the reset after
> > > putting the device off running then the driver state and FW state
> > > become desynchronized.
> > > 
> > > So all the reset paths need to be synchronized some how, either
> > > blocked while in non-running states or aligning the SW state with the
> > > new post-reset FW state.
> > This only catches the two flavors of FLR and the RESET ioctl itself, so
> > we've got gaps relative to "all the reset paths" anyway.  I'm also
> > concerned about adding arbitrary callbacks for every case that it gets
> > too cumbersome to write a wrapper for the existing callbacks.
> > 
> > However, why is this a vfio thing when we have the
> > pci_error_handlers.reset_done callback.  At best this ought to be
> > redundant to that.  Thanks,
> > 
> > Alex
> > 
> Alex,
> 
> How about the below patch instead ?
> 
> This will centralize the 'reset_done' notifications for drivers to one place
> (i.e. pci_error_handlers.reset_done)  and may close the gap that you pointed
> on.
> 
> I just followed the logic in vfio_pci_aer_err_detected() from usage and
> locking point of view.
> 
> Do we really need to take the &vdev->igate mutex as was done there ?
> 
> The next patch from the series in mlx5 will stay as of in V1, it may just
> set its ops and be called upon PCI 'reset_done'.
> 
> 
> diff --git a/drivers/vfio/pci/vfio_pci_core.c
> b/drivers/vfio/pci/vfio_pci_core.c
> index e581a327f90d..20bf37c00fb6 100644
> +++ b/drivers/vfio/pci/vfio_pci_core.c
> @@ -1925,6 +1925,27 @@ static pci_ers_result_t
> vfio_pci_aer_err_detected(struct pci_dev *pdev,
>         return PCI_ERS_RESULT_CAN_RECOVER;
>  }
> 
> +static void vfio_pci_aer_err_reset_done(struct pci_dev *pdev)
> +{
> +       struct vfio_pci_core_device *vdev;
> +       struct vfio_device *device;
> +
> +       device = vfio_device_get_from_dev(&pdev->dev);
> +       if (device == NULL)
> +               return;

Do not add new vfio_device_get_from_dev() calls, this should extract
it from the pci_get_drvdata.

> +
> +       vdev = container_of(device, struct vfio_pci_core_device, vdev);
> +
> +       mutex_lock(&vdev->igate);
> +       if (vdev->ops && vdev->ops->reset_done)
> +               vdev->ops->reset_done(vdev);
> +       mutex_unlock(&vdev->igate);
> +
> +       vfio_device_put(device);
> +
> +       return;
> +}
> +
>  int vfio_pci_core_sriov_configure(struct pci_dev *pdev, int nr_virtfn)
>  {
>         struct vfio_device *device;
> @@ -1947,6 +1968,7 @@ EXPORT_SYMBOL_GPL(vfio_pci_core_sriov_configure);
> 
>  const struct pci_error_handlers vfio_pci_core_err_handlers = {
>         .error_detected = vfio_pci_aer_err_detected,
> +       .reset_done = vfio_pci_aer_err_reset_done,
>  };
>  EXPORT_SYMBOL_GPL(vfio_pci_core_err_handlers);

Most likely mlx5vf should just implement a pci_error_handlers struct
and install vfio_pci_aer_err_detected in it.

Jason