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 <stefa...@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 <faithilike...@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
>
