On Wed, 30 Jan 2019 15:16:15 +0100, Björn Töpel wrote: > Den ons 30 jan. 2019 kl 10:35 skrev Magnus Karlsson: > > > > On Tue, Jan 29, 2019 at 8:15 PM Jakub Kicinski wrote: > > > > > > I may not understand the problem fully, but isn't it kind of normal > > > that if you create a ring empty you'll never receive packets? And it > > > should be reasonably easy to catch while writing an app from scratch > > > (i.e. it behaves deterministically). > > > > Agree that this should be the normal behavior for a NIC. The question > > is how to get out of this situation.There are two options: punt this > > to the application writer or fix this in the driver. I chose to fix > > the driver since this removes complexity in the application. > > > > Magnus' fix addresses a race/timing issue. At zero-copy initialization > point, if the fill ring was empty, the driver (both i40e and ixgbe) > would stop retrying to "allocate" zero-copy frames from the fill > ring. So, frames would never be received, even if the fill ring was > filled at a later point. > > If the driver runs-dry in terms of Rx buffer if one or more frames has > been received, the driver will retry polling the fill-ring. However at > initialization point, if the fill-ring was empty, the driver would > just give up and never retry. > > As Magnus stated, there is no "notify the kernel that the items has > appeared in the fill ring" (other than a HW mechanism where the tail > pointer is a door bell) on the Rx side, so for the Intel drivers it's > up to the driver to solve this.
Oh, I see, so its a missing piece in an otherwise very fault tolerant implementation :) Okay, no objection. The nfp prototype I did simply fails the program load if there are not enough buffers to do the initial fill, but that's just a personal preference. I tend to return errors in unclear situations more than most people.
