On Fri, Jul 29, 2022 at 03:01:51AM +0000, Tian, Kevin wrote: > > From: Jason Gunthorpe <jgg@xxxxxxxxxx> > > Sent: Thursday, July 28, 2022 8:06 PM > > > > On Thu, Jul 28, 2022 at 04:05:04AM +0000, Tian, Kevin wrote: > > > > > > I think this is not correct, just because we made it discoverable does > > > > not absolve the kernel of compatibility. If we change the limit, eg to > > > > 1, and a real userspace stops working then we still broke userspace. > > > > > > iiuc Alex's suggestion doesn't conflict with the 'try and fail' model. > > > By using the reserved field of vfio_device_feature_dma_logging_control > > > to return the limit of the specified page_size from a given tracker, > > > the user can quickly retry and adapt to that limit if workable. > > > > Again, no it can't. The marshalling limit is not the device limit and > > it will still potentially fail. Userspace does not gain much > > additional API certainty by knowing this internal limit. > > Why cannot the tracker return device limit here? Because it is "complicated". mlx5 doesn't really have a limit in simple terms of the number of ranges - it has other limits based on total span and page size. Jason
