qemu/include/io
Leonardo Bras 2bc58ffc29 QIOChannelSocket: Implement io_writev zero copy flag & io_flush for CONFIG_LINUX
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If something like this happen, a newer version of the buffer may be sent instead.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>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20220513062836.965425-4-leobras@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
2022-05-16 13:56:24 +01:00
..
channel-buffer.h io: Fix Lesser GPL version number 2020-10-27 16:48:49 +01:00
channel-command.h io: make qio_channel_command_new_pid() static 2022-05-03 15:47:59 +04:00
channel-file.h io: Fix Lesser GPL version number 2020-10-27 16:48:49 +01:00
channel-socket.h QIOChannelSocket: Implement io_writev zero copy flag & io_flush for CONFIG_LINUX 2022-05-16 13:56:24 +01:00
channel-tls.h io: Fix Lesser GPL version number 2020-10-27 16:48:49 +01:00
channel-util.h io: Fix Lesser GPL version number 2020-10-27 16:48:49 +01:00
channel-watch.h io: Fix Lesser GPL version number 2020-10-27 16:48:49 +01:00
channel-websock.h io: Fix Lesser GPL version number 2020-10-27 16:48:49 +01:00
channel.h QIOChannel: Add flags on io_writev and introduce io_flush callback 2022-05-16 13:56:24 +01:00
dns-resolver.h io: Fix Lesser GPL version number 2020-10-27 16:48:49 +01:00
net-listener.h qom: Remove module_obj_name parameter from OBJECT_DECLARE* macros 2020-09-18 14:12:32 -04:00
task.h io: Fix Lesser GPL version number 2020-10-27 16:48:49 +01:00