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

[Fam Zheng <fam@euphon.net>]

On 2022-04-22 00:36, 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) {
> +        /*
> +         * Only warn about this error: we will fallback to the non-optimized
> +         * io_uring operations.
> +         */
> +        error_reportf_err(*errp,
> +                         "failed to register linux io_uring ring file descriptor");

IIUC errp can be NULL, so let's not dereference it without checking. So, just
use error_report?

Fam

> +    }
>  
> +    return s;
>  }
>  
>  void luring_cleanup(LuringState *s)
> -- 
> Use error_reportf_err to avoid memory leak due to not freeing error
> object.
> --
> 2.35.1
> 
>