On 10/20/21 12:37 PM, Jeff Moyer wrote: > Jens Axboe <axboe@xxxxxxxxx> writes: > >> On 10/20/21 12:16 PM, Jeff Moyer wrote: >>> Hi, Jens, >>> >>> Jens Axboe <axboe@xxxxxxxxx> writes: >>> >>>> It's not used for anything, and we're wasting time passing in zeroes >>>> where we could just ignore it instead. Update all ki_complete users in >>>> the kernel to drop that last argument. >>> >>> What does "wasting time passing in zeroes" mean? >> >> That everybody but the funky usb gadget code passes in zero, hence it's >> a waste of time to pass it in as an argument. > > OK. Just making sure you hadn't found some performance gain from this. > :) Oh there certainly is, not uncommon to see an extra register cleared and used just for passing in this 0. >>> We can't know whether some userspace implementation relies on this >>> behavior, so I don't think you can change it. >> >> Well, I think we should find out, particularly as it's the sole user of >> that extra argument. > > How can we find out? Anyone can write userspace usb gadget code. Some > of those users may be proprietary. Is that likely? I don't know. I'd > rather err on the side of not (potentially) breaking existing > applications, though. Yeah I don't want to risk that either. And since I can see this turning into a discussion on what potentially would be using it, seems like the path of less resistance... Working on just changing it to a 64-bit type instead, then we can pass in both at once with res2 being the upper 32 bits. That'll keep the same API on the aio side. -- Jens Axboe
