On 1/29/20 5:42 PM, Glauber Costa wrote: > > > On Wed, Jan 29, 2020 at 3:55 PM Jens Axboe <axboe@xxxxxxxxx <mailto:axboe@xxxxxxxxx>> wrote: > > On 1/29/20 12:20 PM, Glauber Costa wrote: > > It is common for an application using an ever-evolving interface to want > > to inquire about the presence of certain functionality it plans to use. > > > > The boilerplate to do that is about always the same: find places that > > have feature bits, match that with what we need, rinse, repeat. > > Therefore it makes sense to move this to a library function. > > > > We have two places in which we can check for such features: the feature > > flag returned by io_uring_init_params(), and the resulting array > > returning from io_uring_probe. > > > > I tried my best to communicate as well as possible in the function > > signature the fact that this is not supposed to test the availability > > of io_uring (which is straightforward enough), but rather a minimum set > > of requirements for usage. > > I wonder if we should have a helper that returns the fully allocated > io_uring_probe struct filled out by probing the kernel. My main worry > here is that some applications will probe for various things, each of > which will setup/teardown a ring, and do the query. > > Maybe it'd be enough to potentially pass in a ring? > > > Passing the ring is definitely doable. I think it's important we have both, so that an app can query without having a ring setup. But if it does, we should have the option of using that ring. > While this patch works with a sparse command opcode field, not sure it's > the most natural way. If we do the above, maybe we can just have a > is_this_op_supported() query, since it'd be cheap if we already have the > probe struct filled out? > > > So the user will be the one calling io_register_probe? Not necessarily, I'm thinking something ala: struct io_uring_probe *p p = io_uring_get_probe(); /* call helper functions using 'p' */ free(p); and have io_uring_get_probe_ring() that takes the ring, for example. All depends on what the helpers might be then, I think that's the important part. The rest is just infrastructure to support it. Something like that, hope that makes sense. > Outside of this discussion, some style changes are needed: > > - '*' goes next to the name, struct foo *ptr, not struct foo* ptr > - Some lines over 80 chars > > > Thanks! If you ever feel trapped with the 80 char stuff come write > some c++ seastar code with us! Such a tempting sell, C++ AND long lines ;-) > It's my bad for forgetting, I actually had a last pass on the patch > removing the {} after 1-line ifs so that was fun too No worries. -- Jens Axboe
