On Fri, Sep 9, 2022 at 2:35 PM Jesper Dangaard Brouer <jbrouer@xxxxxxxxxx> wrote: > > > > On 09/09/2022 12.14, Magnus Karlsson wrote: > > On Fri, Sep 9, 2022 at 11:42 AM Jesper Dangaard Brouer > > <jbrouer@xxxxxxxxxx> wrote: > >> > >> > >> On 09/09/2022 10.12, Maryam Tahhan wrote: > >>> <snip> > >>>>>>> > >>>>>>> * Instead encode this information into each metadata entry in the > >>>>>>> metadata area, in some way so that a flags field is not needed (-1 > >>>>>>> signifies not valid, or whatever happens to make sense). This has the > >>>>>>> drawback that the user might have to look at a large number of entries > >>>>>>> just to find out there is nothing valid to read. To alleviate this, it > >>>>>>> could be combined with the next suggestion. > >>>>>>> > >>>>>>> * Dedicate one bit in the options field to indicate that there is at > >>>>>>> least one valid metadata entry in the metadata area. This could be > >>>>>>> combined with the two approaches above. However, depending on what > >>>>>>> metadata you have enabled, this bit might be pointless. If some > >>>>>>> metadata is always valid, then it serves no purpose. But it might if > >>>>>>> all enabled metadata is rarely valid, e.g., if you get an Rx timestamp > >>>>>>> on one packet out of one thousand. > >>>>>>> > >>>>> > >>>>> I like this option better! Except that I have hoped to get 2 bits ;-) > >>>> > >>>> I will give you two if you need it Jesper, no problem :-). > >>>> > >>> > >>> Ok I will look at implementing and testing this and post an update. > >> > >> Perfect if you Maryam have cycles to work on this. > >> > >> Let me explain what I wanted the 2nd bit for. I simply wanted to also > >> transfer the XDP_FLAGS_HINTS_COMPAT_COMMON flag. One could argue that > >> is it redundant information as userspace AF_XDP will have to BTF decode > >> all the know XDP-hints. Thus, it could know if a BTF type ID is > >> compatible with the common struct. This problem is performance as my > >> userspace AF_XDP code will have to do more code (switch/jump-table or > >> table lookup) to map IDs to common compat (to e.g. extract the RX-csum > >> indication). Getting this extra "common-compat" bit is actually a > >> micro-optimization. It is up to AF_XDP maintainers if they can spare > >> this bit. > >> > >> > >>> Thanks folks > >>> > >>>>> The performance advantage is that the AF_XDP descriptor bits will > >>>>> already be cache-hot, and if it indicates no-metadata-hints the AF_XDP > >>>>> application can avoid reading the metadata cache-line :-). > >>>> > >>>> Agreed. I prefer if we can keep it simple and fast like this. > >>>> > >> > >> Great, lets proceed this way then. > >> > >>> <snip> > >>> > >> > >> Thinking ahead: We will likely need 3 bits. > >> > >> The idea is that for TX-side, we set a bit indicating that AF_XDP have > >> provided a valid XDP-hints layout (incl corresponding BTF ID). (I would > >> overload and reuse "common-compat" bit if TX gets a common struct). > > > > I think we should reuse the "Rx metadata valid" flag for this since > > this will not be used in the Tx case by definition. In the Tx case, > > this bit would instead mean that the user has provided a valid > > XDP-hints layout. It has a nice symmetry, on Rx it is set by the > > kernel when it has put something relevant in the metadata area. On Tx, > > it is set by user-space if it has put something relevant in the > > metadata area. > > I generally like reusing the bit, *BUT* there is the problem of > (existing) applications ignoring the desc-options bit and forwarding > packets. This would cause the "Rx metadata valid" flag to be seen as > userspace having set the "TX-hints-bit" and kernel would use what is > provided in metadata area (leftovers from RX-hints). IMHO that will be > hard to debug for end-users and likely break existing applications. Good point. I buy this. We need separate Rx and Tx bits. > > We can also reuse this bit when we get a notification > > in the completion queue to indicate if the kernel has produced some > > metadata on tx completions. This could be a Tx timestamp for example. > > > > Big YES, reuse "Rx metadata valid" bit when we get a TX notification in > completion queue. This will be okay because it cannot be forgotten and > misinterpreted as the kernel will have responsibility to update this bit. > > > So hopefully we could live with only two bits :-). > > > > I still think we need three bits ;-) > That should be enough to cover the 6 states: > - RX hints > - RX hints and compat > - TX hints > - TX hints and compat > - TX completion > - TX completion and compat > > > >> But lets land RX-side first, but make sure we can easily extend for the > >> TX-side. >
