The syntax here needs work; I think my suggestion will work.

Be sure to check with mandoc and igor.

Should add here:

ifp->if_capenable2 = ifp->if_capabilities2;

Maybe 64 Kbytes would be good enough. 2MBytes seems large.

Do we need to update "ifp->if_capenable2" here?

Yes, we do. But this should be done on the cap-by-cap basis, same as it is done for if_capenable. We need to use e.g. mask2 or reuse mask and do similar manipulations.

This is why I did not touched this aspect. Or, do you mean something different?

This value is arbitrary. I do not see a problem with 'too large value' at the moment. For one-off ioctls, it should not matter.

Still missing a space here between NULL and the comma.

Hmm, this is still not addressed. That is, you still won't show "new" capabilities in the capabilities line. capabilities corresponds to if_capabilities whereas options corresponds to if_capenable and both are extended. I think what you want instead is to do something like:

if (ioctl(s, SIOCGIFCAP, ...) == 0) {
    if ((ifr.ifr_curcap & IFCAP_NV != 0) {
      ...
      ioctl(s, SIOCGIFCAPNV, ...);
      ...
      printf("\toptions");
      cookie = NULL;
      for (first = true; ;; first = false) {
          nvname = nvlist_next(...);
          ...
      }
      if (supmedia) {
          printf("\tcapabilities");
          cookie = NULL;
          for (first = true; first = false) {
              nvame = nvlist_next(...);
              if (nvame == NULL) {
                  ...
              }
              if (type == NV_TYPE_BOOL) {
                  printf("%c%s", first ? ' ' : ',', nvname);
              }
          }
          nvlist_destroy(nvcap);
          free(buf);
      }
    } else {
      if (ifr.ifr_curcap != 0) {
          printb("\toptions", ifr.ifr_curcap, IFCAPBITS);
          putchar('\n');
      }
      if (supmedia && ifr.ifr_reqcap != 0) {
          printb("\tcapabilties", ifr.ifr_reqcap, IFCAPBITS);
          putchar('\n');
      }
}

s/JNV/NV/

ENOTTY?

Rather than adding a new ioctl, I would just reuse SIOCSIFCAPNV here and the driver contract would be that you are getting the helper struct in the arg in the driver rather than the user pointer. I think we do similar things in some other network ioctls already, for example ADDMULTI and DELMULTI.

How so? Patch moves the block under the nested 'else'.

IMO there is no point in cross-reference there. We talk not about syscall, but about some feature implemented as part of the described framework.

It is not needed, in the sense that driver code which fills the structure, uses it as well, but this is not a critical path.

Why? Other abbreviations, like 'pcp', 'vlan', or 'fib' are not upper-cased.