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

[Eric Blake <eblake@redhat.com>]

On Sat, Oct 09, 2021 at 04:56:12AM -0300, Leonardo Bras wrote:
> For CONFIG_LINUX, implement the new optional callbacks io_write_zerocopy and
> io_flush_zerocopy on QIOChannelSocket, but enables it only when MSG_ZEROCOPY
> feature is available in the host kernel, which is checked on
> qio_channel_socket_connect_sync()
> 
> qio_channel_socket_writev() contents were moved to a helper function
> qio_channel_socket_writev_flags() which accepts an extra argument for flags.
> (This argument is passed directly to sendmsg().
> 
> The above helper function is used to implement qio_channel_socket_writev(),
> with flags = 0, keeping it's behavior unchanged, and

its (remember, "it's" is shorthand for "it is", which does not fit here)

> qio_channel_socket_writev_zerocopy() with flags = MSG_ZEROCOPY.
> 
> qio_channel_socket_flush_zerocopy() was implemented by counting how many times
> sendmsg(...,MSG_ZEROCOPY) was sucessfully called, and then reading the
> socket's error queue, in order to find how many of them finished sending.
> Flush will loop until those counters are the same, or until some error ocurs.

occurs

> 
> A new function qio_channel_socket_poll() was also created in order to avoid
> busy-looping recvmsg() in qio_channel_socket_flush_zerocopy() while waiting for
> updates in socket's error queue.
> 
> Notes on using writev_zerocopy():
> 1: Buffer
> - As MSG_ZEROCOPY tells the kernel to use the same user buffer to avoid copying,
> some caution is necessary to avoid overwriting any buffer before it's sent.
> If something like this happen, a newer version of the buffer may be sent instead.
> - If this is a problem, it's recommended to call flush_zerocopy() before freeing
> or re-using the buffer.
> 
> 2: Locked memory
> - When using MSG_ZERCOCOPY, the buffer memory will be locked after queued, and
> unlocked after it's sent.
> - Depending on the size of each buffer, and how often it's sent, it may require
> a larger amount of locked memory than usually available to non-root user.
> - If the required amount of locked memory is not available, writev_zerocopy
> will return an error, which can abort an operation like migration,
> - Because of this, when an user code wants to add zerocopy as a feature, it
> requires a mechanism to disable it, so it can still be acessible to less
> privileged users.
> 
> Signed-off-by: Leonardo Bras <leobras@redhat.com>
> ---
>  include/io/channel-socket.h |   2 +
>  include/io/channel.h        |   1 +
>  io/channel-socket.c         | 180 ++++++++++++++++++++++++++++++++++--
>  3 files changed, 173 insertions(+), 10 deletions(-)
> 
> +static int qio_channel_socket_flush_zerocopy(QIOChannel *ioc,
> +                                             Error **errp)
> +{

> +
> +        /* No errors, count sucessfully finished sendmsg()*/

Space before */

> +        sioc->zerocopy_sent += serr->ee_data - serr->ee_info + 1;
> +    }
> +    return 0;
> +}
> +
> +#endif /* CONFIG_LINUX */
> +
>  static int
>  qio_channel_socket_set_blocking(QIOChannel *ioc,
>                                  bool enabled,
> @@ -787,6 +943,10 @@ static void qio_channel_socket_class_init(ObjectClass *klass,
>      ioc_klass->io_set_delay = qio_channel_socket_set_delay;
>      ioc_klass->io_create_watch = qio_channel_socket_create_watch;
>      ioc_klass->io_set_aio_fd_handler = qio_channel_socket_set_aio_fd_handler;
> +#ifdef CONFIG_LINUX
> +    ioc_klass->io_writev_zerocopy = qio_channel_socket_writev_zerocopy;
> +    ioc_klass->io_flush_zerocopy = qio_channel_socket_flush_zerocopy;
> +#endif
>  }

I did a high-level look at the code, rather than an in-depth review of
whether zero-copy was being used correctly.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org