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

[Daniel P . Berrangé]

On Fri, Apr 22, 2022 at 09:34:28AM +0100, Fam Zheng wrote:
> 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?

Plenty of people will be running kernels that lack the new feature,
so this "failure" will be an expected scenario. We shouldn't be
spamming the logs with any error or warning message. Assuming  QEMU
remains fully functional, merely not as optimized, we should be
totally silent.

At most stick in a 'trace' point so we can record whether the
optimization is present.

With regards,
Daniel
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