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SubjectRe: [PATCH] PCI: Add link_change error handler and vfio-pci user
On Wed, 24 Apr 2019 16:45:45 +0000
<Alex_Gagniuc@Dellteam.com> wrote:

> On 4/23/2019 5:42 PM, Alex Williamson wrote:
> > The PCIe bandwidth notification service generates logging any time a
> > link changes speed or width to a state that is considered downgraded.
> > Unfortunately, it cannot differentiate signal integrity related link
> > changes from those intentionally initiated by an endpoint driver,
> > including drivers that may live in userspace or VMs when making use
> > of vfio-pci. Therefore, allow the driver to have a say in whether
> > the link is indeed downgraded and worth noting in the log, or if the
> > change is perhaps intentional.
> >
> > For vfio-pci, we don't know the intentions of the user/guest driver
> > either, but we do know that GPU drivers in guests actively manage
> > the link state and therefore trigger the bandwidth notification for
> > what appear to be entirely intentional link changes.
> >
> > Fixes: e8303bb7a75c PCI/LINK: Report degraded links via link bandwidth notification
> > Link: https://lore.kernel.org/linux-pci/155597243666.19387.1205950870601742062.stgit@gimli.home/T/#u
> > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> > ---
> >
> > Changing to pci_dbg() logging is not super usable, so let's try the
> > previous idea of letting the driver handle link change events as they
> > see fit. Ideally this might be two patches, but for easier handling,
> > folding the pci and vfio-pci bits together. Comments? Thanks,
>
> I think this callback opens up a can of worms where drivers can ad-hoc
> kill a number what otherwise can be indicators of problems. But I don't
> have to like it to review it :).
>
> > drivers/pci/probe.c | 13 +++++++++++++
> > drivers/vfio/pci/vfio_pci.c | 10 ++++++++++
> > include/linux/pci.h | 3 +++
> > 3 files changed, 26 insertions(+)
> >
> > diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> > index 7e12d0163863..233cd4b5b6e8 100644
> > --- a/drivers/pci/probe.c
> > +++ b/drivers/pci/probe.c
> > @@ -2403,6 +2403,19 @@ void pcie_report_downtraining(struct pci_dev *dev)
>
> I don't think you want to change pcie_report_downtraining(). You're
> advertising to "report" something, by nomenclature, but then go around
> and also call a notification callback. This is also used during probe,
> and you've now just killed your chance to notice you've booted with a
> degraded link.
> If what you want to do is silence the bandwidth notification, you want
> to modify the threaded interrupt that calls this.

During probe, ie. discovery, a device wouldn't have a driver attached,
so we'd fall through to simply printing the link status. Nothing
lost afaict. The "report" verb doesn't have a subject here, report to
whom? Therefore I thought it reasonable that a driver ask that it be
reported to them via a callback. I don't see that as such a stretch of
the interface.

> > if (PCI_FUNC(dev->devfn) != 0 || dev->is_virtfn)
> > return;
> >
> > + /*
> > + * If driver handles link_change event, defer to driver. PCIe drivers
> > + * can call pcie_print_link_status() to print current link info.
> > + */
> > + device_lock(&dev->dev);
> > + if (dev->driver && dev->driver->err_handler &&
> > + dev->driver->err_handler->link_change) {
> > + dev->driver->err_handler->link_change(dev);
> > + device_unlock(&dev->dev);
> > + return;
> > + }
> > + device_unlock(&dev->dev);
>
> Can we write this such that there is a single lock()/unlock() pair?

Not without introducing a tracking variable, ex.

bool handled = false;

lock()
if (stuff) {
link_change()
handled = true;
}
unlock()
if (!handled)
dmesg spew
That's not markedly better imo, but if it's preferred I can send a v2.
Thanks,

Alex

> > +
> > /* Print link status only if the device is constrained by the fabric */
> > __pcie_print_link_status(dev, false);
> > }
> > diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
> > index cab71da46f4a..c9ffc0ccabb3 100644
> > --- a/drivers/vfio/pci/vfio_pci.c
> > +++ b/drivers/vfio/pci/vfio_pci.c
> > @@ -1418,8 +1418,18 @@ static pci_ers_result_t vfio_pci_aer_err_detected(struct pci_dev *pdev,
> > return PCI_ERS_RESULT_CAN_RECOVER;
> > }
> >
> > +/*
> > + * Ignore link change notification, we can't differentiate signal related
> > + * link changes from user driver power management type operations, so do
> > + * nothing. Potentially this could be routed out to the user.
> > + */
> > +static void vfio_pci_link_change(struct pci_dev *pdev)
> > +{
> > +}
> > +
> > static const struct pci_error_handlers vfio_err_handlers = {
> > .error_detected = vfio_pci_aer_err_detected,
> > + .link_change = vfio_pci_link_change,
> > };
> >
> > static struct pci_driver vfio_pci_driver = {
> > diff --git a/include/linux/pci.h b/include/linux/pci.h
> > index 27854731afc4..e9194bc03f9e 100644
> > --- a/include/linux/pci.h
> > +++ b/include/linux/pci.h
> > @@ -763,6 +763,9 @@ struct pci_error_handlers {
> >
> > /* Device driver may resume normal operations */
> > void (*resume)(struct pci_dev *dev);
> > +
> > + /* PCIe link change notification */
> > + void (*link_change)(struct pci_dev *dev);
> > };
> >
> >
> >
> >
>
>

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