Re: [PATCH v4 2/3] QIOChannelSocket: Implement io_writev_zerocopy & io_flush_zerocopy for CONFIG_LINUX

[Leonardo Bras Soares Passos <leobras@redhat.com>]

On Wed, Oct 13, 2021 at 3:18 AM Peter Xu <peterx@redhat.com> wrote:
>
> On Sat, Oct 09, 2021 at 04:56:12AM -0300, Leonardo Bras wrote:
> > @@ -154,6 +161,17 @@ int qio_channel_socket_connect_sync(QIOChannelSocket *ioc,
> >          return -1;
> >      }
> >
> > +#ifdef CONFIG_LINUX
> > +    ret = qemu_setsockopt(fd, SOL_SOCKET, SO_ZEROCOPY, &v, sizeof(v));
> > +    if (ret < 0) {
> > +        /* Zerocopy not available on host */
> > +        return 0;
> > +    }
> > +
> > +    qio_channel_set_feature(QIO_CHANNEL(ioc),
> > +                            QIO_CHANNEL_FEATURE_WRITE_ZEROCOPY);
>
> This is okay I think, but looks a bit weird.  Maybe nicer to be written as:
>
> #if LINUX
>       ret = setsockopt();
>       if (ret == 0) {
>           qio_channel_set_feature(...);
>       }
> #endif
>       return 0;
>
> ?

Yeah, I also questioned myself about this one.
At the time I ended up writing like this because the lines above used
the behavior "if error, then exit/abort", and so I thought that this
would be the better way to include this feature.
But I did not consider that this is not an error exit, but a 'maybe
feature instead'.

So, I will change that like you suggested.

>
> > +#endif
> > +
> >      return 0;
> >  }
>
> [...]
>
> > +static ssize_t qio_channel_socket_writev_zerocopy(QIOChannel *ioc,
> > +                                                  const struct iovec *iov,
> > +                                                  size_t niov,
> > +                                                  Error **errp)
> > +{
> > +    QIOChannelSocket *sioc = QIO_CHANNEL_SOCKET(ioc);
> > +    ssize_t ret;
> > +
> > +    ret = qio_channel_socket_writev_flags(ioc, iov, niov, NULL, 0,
> > +                                          MSG_ZEROCOPY, errp);
> > +    if (ret == QIO_CHANNEL_ERR_NOBUFS) {
> > +        if (errp && *errp) {
>
> Hmm this seems wrong, *errp should be NULL in most cases, meanwhile I think
> error_setg*() takes care of errp==NULL too, so maybe we can drop this?

Yeah, you are correct.
I ended up confused about how to use err, thanks for making it more clear!

>
> > +            error_setg_errno(errp, errno,
> > +                             "Process can't lock enough memory for using MSG_ZEROCOPY");
> > +        }
> > +        return -1;
> > +    }
> > +
> > +    sioc->zerocopy_queued++;
> > +    return ret;
> > +}
>
> --
> Peter Xu
>

Best regards,
Leonardo Bras