Re: [PATCH v4] Use io_uring_register_ring_fd() to skip fd operations

[olc]

Hi Stefan,

I've tested the code and it behaves as you expected. Should I add this to a new patch version or leave it as is?

Sam

Stefan Hajnoczi <stefanha@gmail.com> 于2022年4月22日周五 23:10写道：

On Fri, Apr 22, 2022 at 12:36:49AM +0800, Sam Li wrote:
> Linux recently added a new io_uring(7) optimization API that QEMU
> doesn't take advantage of yet. The liburing library that QEMU uses
> has added a corresponding new API calling io_uring_register_ring_fd().
> When this API is called after creating the ring, the io_uring_submit()
> library function passes a flag to the io_uring_enter(2) syscall
> allowing it to skip the ring file descriptor fdget()/fdput()
> operations. This saves some CPU cycles.
>
> Signed-off-by: Sam Li <faithilikerun@gmail.com>
> ---
>  block/io_uring.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/block/io_uring.c b/block/io_uring.c
> index 782afdb433..5247fb79e2 100644
> --- a/block/io_uring.c
> +++ b/block/io_uring.c
> @@ -435,8 +435,16 @@ LuringState *luring_init(Error **errp)
>      }
> 
>      ioq_init(&s->io_q);
> -    return s;
> +    if (io_uring_register_ring_fd(&s->ring) < 0) {

What happens when QEMU is built against an older version of liburing
that lacks the io_uring_register_ring_fd() API?

I guess there will be a compiler error because the function prototype is
missing in <liburing.h>.

This can be addressed by checking for the presence of the function in
meson.build:

+config_host_data.set('CONFIG_LIBURING_REGISTER_RING_FD', cc.has_function('io_uring_register_ring_fd', prefix: '#include <liburing.h>'))

Then block/io_uring.c can call the function only when available:

+#ifdef CONFIG_LIBURING_REGISTER_RING_FD
+    io_uring_register_ring_fd(&s->ring);
+#endif

(I haven't tested this code but it should be close.)

Stefan