Re: [PATCH 05/16] Add io_uring IO interface

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 05/16] Add io_uring IO interface
Date: Wed, 16 Jan 2019 16:16:47 +0100	[thread overview]
Message-ID: <CAK8P3a2MuO-wRjYxscP-AOAUjwg6fHRc99Am6xqUmXYrJRt44w@mail.gmail.com> (raw)
In-Reply-To: <15b06ccb-4501-f104-9a3a-f028218a0d70@kernel.dk>

On Wed, Jan 16, 2019 at 4:12 PM Jens Axboe <axboe@kernel.dk> wrote:
> On 1/16/19 3:41 AM, Arnd Bergmann wrote:
> > On Tue, Jan 15, 2019 at 3:55 AM Jens Axboe <axboe@kernel.dk> wrote:
> >> diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
> >> index 3cf7b533b3d1..194e79c0032e 100644
> >> --- a/arch/x86/entry/syscalls/syscall_32.tbl
> >> +++ b/arch/x86/entry/syscalls/syscall_32.tbl
> >> +SYSCALL_DEFINE2(io_uring_setup, u32, entries,
> >> +               struct io_uring_params __user *, params)
> >> +{
> >> +       return io_uring_setup(entries, params, false);
> >> +}
> >> +
> >> +#ifdef CONFIG_COMPAT
> >> +COMPAT_SYSCALL_DEFINE2(io_uring_setup, u32, entries,
> >> +                      struct io_uring_params __user *, params)
> >> +{
> >> +       return io_uring_setup(entries, params, true);
> >> +}
> >> +#endif
> >
> > The compat syscall has the same calling conventions as the
> > native one here, so I think you can just use that directly.
>
> Not sure I understand what you mean here. I need to know if it's the
> compat one, hence 'true' vs 'false', so I know what the size of the user
> pointers/structs are.

My mistake, I missed the true/false difference between the two
functions.

> >> +/*
> >> + * IO submission data structure (Submission Queue Entry)
> >> + */
> >> +struct io_uring_sqe {
> >> +       __u8    opcode;         /* type of operation for this sqe */
> >> +       __u8    flags;          /* as of now unused */
> >> +       __u16   ioprio;         /* ioprio for the request */
> >> +       __s32   fd;             /* file descriptor to do IO on */
> >> +       __u64   off;            /* offset into file */
> >> +       union {
> >> +               void    *addr;  /* buffer or iovecs */
> >> +               __u64   __pad;
> >> +       };
> >
> > It seems a bit unfortunate to keep the pointer field only
> > almost compatible between 32-bit and 64-bit big-endian
> > architectures, as that requires an in_compat_syscall()
> > check whenever we access the pointer from the kernel.
> >
> > Could you use a __u64 field to store the pointer itself
> > instead?
>
> I feel like I'm missing something here, we'll still need the compat code
> on the kernel side for 32-bit app on 64-bit kernel, so what would we
> solve by making this an __u64?

It means you don't have to define a compat_io_uring_sqe
structure with a compat_uptr_t member in it.

      Arnd

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