On Mon, Apr 29, 2019 at 08:51:04AM -0600, Alex Williamson wrote: > On Wed, 24 Apr 2019 12:57:58 -0500 > Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote: > > > On Tue, Apr 23, 2019 at 04:42:28PM -0600, 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@xxxxxxxxxx/T/#u > > > Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx> > > > --- > > > > > > 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'm a little uneasy about the bandwidth notification logging as a > > whole. Messages in dmesg don't seem like a solid base for building > > management tools. > > > > I assume the eventual goal would be some sort of digested notification > > along the lines of "hey mr/ms administrator, the link to device X > > unexpectedly became slower, you might want to check that out." > > > > If I were building that, I don't think I would use dmesg. I might > > write a daemon that polls /sys/.../current_link_{speed,width}, or > > maybe use some sort of netlink event. Maybe it would be useful to > > have the admin designate devices of interest. > > > > I'm hesitant about adding a .link_change() handler. If there's > > something useful a driver could do with it, that's one thing. But > > using it merely to suppress a message doesn't really seem worth the > > trouble, and it seems unfriendly to ask drivers to add it when they > > didn't ask for it and get no benefit from it. > > So where do we go from here? I agree that dmesg is not necessarily a > great choice for these sorts of events and if they went somewhere else, > maybe I wouldn't have the same concerns about them generating user > confusion or contributing to DoS vectors from userspace drivers. As it > is though, we have known cases where benign events generate confusing > logging messages, which seems like a regression. Drivers didn't ask > for a link_change handler, but nor did they ask that the link state to > their device be monitored so closely. Maybe this not only needs some > sort of change to the logging mechanism, but also an opt-in by the > driver if they don't expect runtime link changes. Thanks, I think it's really too late in the cycle to rework this and get changes merged before the v5.1 release (probably on May 5), so I'll queue up a revert and we can iron out the wrinkles for v5.2. Bjorn
