On Thu, 13 Apr 2017 11:37:22 -0400 (EDT) David Miller <da...@davemloft.net> wrote:
> From: Alexei Starovoitov <alexei.starovoi...@gmail.com> > Date: Wed, 12 Apr 2017 21:20:38 -0700 > > > On Wed, Apr 12, 2017 at 02:54:15PM -0400, David Miller wrote: [...] > > If the capability is variable, it must be communicated to the user > somehow at program load time. > > We are consistently finding that there is this real need to > communicate XDP capabilities, or somehow verify that the needs > of an XDP program can be satisfied by a given implementation. I fully agree that we need some way to express capabilities[1] [1] http://prototype-kernel.readthedocs.io/en/latest/networking/XDP/design/design.html#capabilities-negotiation > Maximum headroom is just one. [...] > > We can only optimize this and elide things when we have a facility in > the future for the program to express it's needs precisely. I think > we will have to add some control structure to XDP programs that can > be filled in for this purpose. I fully agree that we need some control structure to XDP programs. My previous attempt was shot-down due to performance concerns of an extra pointer dereference. As I explained before, this is not a concern as the dereference will happen once per N packets in the NAPI loop. Plus now we see a need to elide things based on facilities the XDP program choose to use/enable, for performance reasons. I would prefer keeping these facility settings in control structure to XDP programs, instead of pulling in derived bits runtime. Again remember, adding if/branch statements checking for facilities, should have little performance impact as the branch predictor should guess correctly given we process N packets in the NAPI loop with same facilities. -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer
