Re: [PATCH v11 2/7] QIOChannelSocket: Implement io_writev zero copy flag & io_flush for CONFIG_LINUX

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, May 04, 2022 at 04:18:31PM -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 If something like this happen, a newer version of the buffer may be sent instead.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>
> Reviewed-by: Peter Xu <peterx@redhat.com>
> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
> Reviewed-by: Juan Quintela <quintela@redhat.com>
> ---
>  include/io/channel-socket.h |   2 +
>  io/channel-socket.c         | 120 ++++++++++++++++++++++++++++++++++--
>  2 files changed, 118 insertions(+), 4 deletions(-)
> 
> diff --git a/include/io/channel-socket.h b/include/io/channel-socket.h
> index e747e63514..513c428fe4 100644
> --- a/include/io/channel-socket.h
> +++ b/include/io/channel-socket.h
> @@ -47,6 +47,8 @@ struct QIOChannelSocket {
>      socklen_t localAddrLen;
>      struct sockaddr_storage remoteAddr;
>      socklen_t remoteAddrLen;
> +    ssize_t zero_copy_queued;
> +    ssize_t zero_copy_sent;
>  };
>  
>  
> diff --git a/io/channel-socket.c b/io/channel-socket.c
> index 696a04dc9c..ae756ce166 100644
> --- a/io/channel-socket.c
> +++ b/io/channel-socket.c
> @@ -25,9 +25,25 @@
>  #include "io/channel-watch.h"
>  #include "trace.h"
>  #include "qapi/clone-visitor.h"
> +#ifdef CONFIG_LINUX
> +#include <linux/errqueue.h>
> +#include <sys/socket.h>
> +#endif
>  
>  #define SOCKET_MAX_FDS 16
>  
> +/*
> + * Zero-copy defines bellow are included to avoid breaking builds on systems
> + * that don't support MSG_ZEROCOPY, while keeping the functions more readable
> + * (without a lot of ifdefs).
> + */
> +#ifndef MSG_ZEROCOPY
> +#define MSG_ZEROCOPY 0x4000000
> +#endif
> +#ifndef SO_ZEROCOPY
> +#define SO_ZEROCOPY 60
> +#endif

Please put these behind CONFIG_LINUX to make it clear to readers that
this is entirely Linux specific


> +
>  SocketAddress *
>  qio_channel_socket_get_local_address(QIOChannelSocket *ioc,
>                                       Error **errp)
> @@ -54,6 +70,8 @@ qio_channel_socket_new(void)
>  
>      sioc = QIO_CHANNEL_SOCKET(object_new(TYPE_QIO_CHANNEL_SOCKET));
>      sioc->fd = -1;
> +    sioc->zero_copy_queued = 0;
> +    sioc->zero_copy_sent = 0;
>  
>      ioc = QIO_CHANNEL(sioc);
>      qio_channel_set_feature(ioc, QIO_CHANNEL_FEATURE_SHUTDOWN);
> @@ -153,6 +171,16 @@ int qio_channel_socket_connect_sync(QIOChannelSocket *ioc,
>          return -1;
>      }
>  
> +#ifdef CONFIG_LINUX
> +    int ret, v = 1;
> +    ret = setsockopt(fd, SOL_SOCKET, SO_ZEROCOPY, &v, sizeof(v));
> +    if (ret == 0) {
> +        /* Zero copy available on host */
> +        qio_channel_set_feature(QIO_CHANNEL(ioc),
> +                                QIO_CHANNEL_FEATURE_WRITE_ZERO_COPY);
> +    }
> +#endif
> +
>      return 0;
>  }
>  
> @@ -533,6 +561,7 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
>      char control[CMSG_SPACE(sizeof(int) * SOCKET_MAX_FDS)];
>      size_t fdsize = sizeof(int) * nfds;
>      struct cmsghdr *cmsg;
> +    int sflags = 0;
>  
>      memset(control, 0, CMSG_SPACE(sizeof(int) * SOCKET_MAX_FDS));
>  
> @@ -557,15 +586,27 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
>          memcpy(CMSG_DATA(cmsg), fds, fdsize);
>      }
>  
> +    if (flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY) {
> +        sflags = MSG_ZEROCOPY;
> +    }

Also should be behind CONFIG_LINUX

> +
>   retry:
> -    ret = sendmsg(sioc->fd, &msg, 0);
> +    ret = sendmsg(sioc->fd, &msg, sflags);
>      if (ret <= 0) {
> -        if (errno == EAGAIN) {
> +        switch (errno) {
> +        case EAGAIN:
>              return QIO_CHANNEL_ERR_BLOCK;
> -        }
> -        if (errno == EINTR) {
> +        case EINTR:
>              goto retry;
> +        case ENOBUFS:
> +            if (sflags & MSG_ZEROCOPY) {
> +                error_setg_errno(errp, errno,
> +                                 "Process can't lock enough memory for using MSG_ZEROCOPY");
> +                return -1;
> +            }
> +            break;

And this ENOBUGS case behind CONFIG_LINUX

>          }
> +
>          error_setg_errno(errp, errno,
>                           "Unable to write to socket");
>          return -1;
> @@ -659,6 +700,74 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
>  }
>  #endif /* WIN32 */
>  
> +
> +#ifdef CONFIG_LINUX
> +static int qio_channel_socket_flush(QIOChannel *ioc,
> +                                    Error **errp)
> +{
> +    QIOChannelSocket *sioc = QIO_CHANNEL_SOCKET(ioc);
> +    struct msghdr msg = {};
> +    struct sock_extended_err *serr;
> +    struct cmsghdr *cm;
> +    char control[CMSG_SPACE(sizeof(*serr))];
> +    int received;
> +    int ret = 1;
> +
> +    msg.msg_control = control;
> +    msg.msg_controllen = sizeof(control);
> +    memset(control, 0, sizeof(control));
> +
> +    while (sioc->zero_copy_sent < sioc->zero_copy_queued) {
> +        received = recvmsg(sioc->fd, &msg, MSG_ERRQUEUE);
> +        if (received < 0) {
> +            switch (errno) {
> +            case EAGAIN:
> +                /* Nothing on errqueue, wait until something is available */
> +                qio_channel_wait(ioc, G_IO_ERR);
> +                continue;
> +            case EINTR:
> +                continue;
> +            default:
> +                error_setg_errno(errp, errno,
> +                                 "Unable to read errqueue");
> +                return -1;
> +            }
> +        }
> +
> +        cm = CMSG_FIRSTHDR(&msg);
> +        if (cm->cmsg_level != SOL_IP &&
> +            cm->cmsg_type != IP_RECVERR) {
> +            error_setg_errno(errp, EPROTOTYPE,
> +                             "Wrong cmsg in errqueue");
> +            return -1;
> +        }
> +
> +        serr = (void *) CMSG_DATA(cm);
> +        if (serr->ee_errno != SO_EE_ORIGIN_NONE) {
> +            error_setg_errno(errp, serr->ee_errno,
> +                             "Error on socket");
> +            return -1;
> +        }
> +        if (serr->ee_origin != SO_EE_ORIGIN_ZEROCOPY) {
> +            error_setg_errno(errp, serr->ee_origin,
> +                             "Error not from zero copy");
> +            return -1;
> +        }
> +
> +        /* No errors, count successfully finished sendmsg()*/
> +        sioc->zero_copy_sent += serr->ee_data - serr->ee_info + 1;
> +
> +        /* If any sendmsg() succeeded using zero copy, return 0 at the end */
> +        if (serr->ee_code != SO_EE_CODE_ZEROCOPY_COPIED) {
> +            ret = 0;
> +        }
> +    }
> +
> +    return ret;
> +}
> +
> +#endif /* CONFIG_LINUX */
> +
>  static int
>  qio_channel_socket_set_blocking(QIOChannel *ioc,
>                                  bool enabled,
> @@ -789,6 +898,9 @@ 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_flush = qio_channel_socket_flush;
> +#endif
>  }
>  
>  static const TypeInfo qio_channel_socket_info = {
> -- 
> 2.36.0
> 

With regards,
Daniel
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