Re: [PATCH v2] io_uring: add support for probing opcodes

[Jens Axboe <axboe@kernel.dk>]

On 1/17/20 10:15 AM, Jeff Moyer wrote:
> Jens Axboe <axboe@kernel.dk> writes:
> 
>> The application currently has no way of knowing if a given opcode is
>> supported or not without having to try and issue one and see if we get
>> -EINVAL or not. And even this approach is fraught with peril, as maybe
>> we're getting -EINVAL due to some fields being missing, or maybe it's
>> just not that easy to issue that particular command without doing some
>> other leg work in terms of setup first.
>>
>> This adds IORING_REGISTER_PROBE, which fills in a structure with info
>> on what it supported or not. This will work even with sparse opcode
>> fields, which may happen in the future or even today if someone
>> backports specific features to older kernels.
> 
> This looks pretty good to me.  You can call it with 0 args to get the
> total number of ops, then allocate an array with that number and
> re-issue the syscall.  I also like that you've allowed for backporting
> subsets of functionality.

Right, this is similar to how most hardware commands work when you don't
know what the max size would be. Since this is pretty small, I would
expect applications to just use 256 as the value and get all of them.
But if they want to probe and use that method, that'll work just fine.

> I have one question below:
> 
>> @@ -6632,6 +6674,12 @@ static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
>>  			break;
>>  		ret = io_eventfd_unregister(ctx);
>>  		break;
>> +	case IORING_REGISTER_PROBE:
>> +		ret = -EINVAL;
>> +		if (!arg || nr_args > 256)
>> +			break;
>> +		ret = io_probe(ctx, arg, nr_args);
>> +		break;
> 
> Why 256?  If it's just arbitrary, please add a comment.

We can't have more than 256 opcodes, as it's a byte for the opcode.

> Otherwise looks good!
> 
> Reviewed-by: Jeff Moyer <jmoyer@redhat.com>

Thanks!

-- 
Jens Axboe