Re: [PATCH 13/16] io_uring: add support for pre-mapped user IO buffers

From: Arnd Bergmann <arnd@arndb.de>
To: Jens Axboe <axboe@kernel.dk>
Cc: Linux FS-devel Mailing List <linux-fsdevel@vger.kernel.org>,
	linux-aio <linux-aio@kvack.org>,
	linux-block <linux-block@vger.kernel.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	Christoph Hellwig <hch@lst.de>, Jeff Moyer <jmoyer@redhat.com>,
	Avi Kivity <avi@scylladb.com>
Subject: Re: [PATCH 13/16] io_uring: add support for pre-mapped user IO buffers
Date: Wed, 16 Jan 2019 16:41:12 +0100	[thread overview]
Message-ID: <CAK8P3a2E8dLoL5sgucp=-LDkGMN=qnG8M_phHoHCBg5ipWgBjQ@mail.gmail.com> (raw)
In-Reply-To: <331f63c0-002f-b373-b831-73af9e98f2ef@kernel.dk>

On Wed, Jan 16, 2019 at 4:32 PM Jens Axboe <axboe@kernel.dk> wrote:
>
> On 1/16/19 8:14 AM, Jens Axboe wrote:
> > On 1/16/19 3:53 AM, Arnd Bergmann wrote:
> >> On Tue, Jan 15, 2019 at 3:56 AM Jens Axboe <axboe@kernel.dk> wrote:
> >>
> >>> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> >>> index 542757a4c898..e36c264d74e8 100644
> >>> --- a/include/linux/syscalls.h
> >>> +++ b/include/linux/syscalls.h
> >>> @@ -314,6 +314,8 @@ asmlinkage long sys_io_uring_setup(u32 entries,
> >>>                                 struct io_uring_params __user *p);
> >>>  asmlinkage long sys_io_uring_enter(unsigned int fd, u32 to_submit,
> >>>                                 u32 min_complete, u32 flags);
> >>> +asmlinkage long sys_io_uring_register(unsigned int fd, unsigned op,
> >>> +                               void __user *arg);
> >>>
> >>
> >> Would it be possible to make this a typed pointer instead? If this needs to
> >> be extended later to pass a different structure, a new system call may
> >> be better for consistency than overloading the argument in various
> >> ways.
> >
> > As you can see from the later patch for registering files, it'll be used
> > for other structs too. Feels a little silly to add an extra system call
> > for that. I agree the void * isn't the prettiest thing in the world, but
> > at least it allows us to extend the API without having to add even more
> > system calls down the line.
>
> With the __u64 changes, we end up with this:
>
> struct io_uring_register_buffers {
>         __u64 iovecs;           /* pointer to iovecs array */
>         __u32 nr_iovecs;        /* number of iovecs in array */
>         __u32 pad;
> };
>
> struct io_uring_register_files {
>         __u64 fds;
>         __u32 nr_fds;
>         __u32 pad;
> };
>
> which are identical. So the question then becomes if I should just make
> these opaque enough to be the same thing, ala:
>
> struct io_uring_register_data {
>         __u64 data;
>         __u32 nr_elems;
>         __u32 pad;
> };

Right, that looks good in either form.

> and then probably add a bit more reserved space so we have something
> that can be extended...

Or maybe go the opposite way and pass the two members you have
directly to the system call:

int io_uring_register(unsigned int fd, unsigned int opcode, void
__user *, arg, unsigned count)
{
      ...
}

Where 'arg' now points to the array of iovecs or the the array of file
descriptors, or whatever else you need.

       Arnd