Re: [PATCH v7 2/5] QIOChannelSocket: Implement io_writev zero copy flag & io_flush for CONFIG_LINUX

[Daniel P . Berrangé]

On Thu, Jan 06, 2022 at 07:13:39PM -0300, Leonardo Bras wrote:
> For CONFIG_LINUX, implement the new zero copy flag and the optional callback
> io_flush 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_flush() was implemented by counting how many times
> sendmsg(...,MSG_ZEROCOPY) was successfully 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 occurs.
> 
> Notes on using writev() with QIO_CHANNEL_WRITE_FLAG_ZERO_COPY:
> 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 If something like this happen, a newer version of the buffer may be sent 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 qio_channel_flush() 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_zero_copy
> will return an error, which can abort an operation like migration,
> - Because of this, when an user code wants to add zero copy as a feature, it
> requires a mechanism to disable it, so it can still be accessible to less
> privileged users.
> 
> Signed-off-by: Leonardo Bras <leobras@redhat.com>
> ---
>  include/io/channel-socket.h |   2 +
>  io/channel-socket.c         | 107 ++++++++++++++++++++++++++++++++++--
>  2 files changed, 105 insertions(+), 4 deletions(-)

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>


Regards,
Daniel
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