Re: [PATCH] migration: check magic value for deciding the mapping of channels

[Daniel P . Berrangé]

On Tue, Nov 01, 2022 at 02:30:29PM +0000, manish.mishra wrote:
> Current logic assumes that channel connections on the destination side are
> always established in the same order as the source and the first one will
> always be the default channel followed by the multifid or post-copy
> preemption channel. This may not be always true, as even if a channel has a
> connection established on the source side it can be in the pending state on
> the destination side and a newer connection can be established first.
> Basically causing out of order mapping of channels on the destination side.
> Currently, all channels except post-copy preempt send a magic number, this
> patch uses that magic number to decide the type of channel. This logic is
> applicable only for precopy(multifd) live migration, as mentioned, the
> post-copy preempt channel does not send any magic number. Also, this patch
> uses MSG_PEEK to check the magic number of channels so that current
> data/control stream management remains un-effected.
> 
> Signed-off-by: manish.mishra <manish.mishra@nutanix.com>
> ---
>  include/io/channel.h     | 25 +++++++++++++++++++++++++
>  io/channel-socket.c      | 27 +++++++++++++++++++++++++++
>  io/channel.c             | 39 +++++++++++++++++++++++++++++++++++++++
>  migration/migration.c    | 33 +++++++++++++++++++++------------
>  migration/multifd.c      | 12 ++++--------
>  migration/multifd.h      |  2 +-
>  migration/postcopy-ram.c |  5 +----
>  migration/postcopy-ram.h |  2 +-
>  8 files changed, 119 insertions(+), 26 deletions(-)
> 
> diff --git a/include/io/channel.h b/include/io/channel.h
> index c680ee7480..74177aeeea 100644
> --- a/include/io/channel.h
> +++ b/include/io/channel.h
> @@ -115,6 +115,10 @@ struct QIOChannelClass {
>                          int **fds,
>                          size_t *nfds,
>                          Error **errp);
> +    ssize_t (*io_read_peek)(QIOChannel *ioc,
> +                            void *buf,
> +                            size_t nbytes,
> +                            Error **errp);
>      int (*io_close)(QIOChannel *ioc,
>                      Error **errp);
>      GSource * (*io_create_watch)(QIOChannel *ioc,
> @@ -475,6 +479,27 @@ int qio_channel_write_all(QIOChannel *ioc,
>                            size_t buflen,
>                            Error **errp);
>  
> +/**
> + * qio_channel_read_peek_all:
> + * @ioc: the channel object
> + * @buf: the memory region to read in data
> + * @nbytes: the number of bytes to read
> + * @errp: pointer to a NULL-initialized error object
> + *
> + * Read given @nbytes data from peek of channel into
> + * memory region @buf.
> + *
> + * The function will be blocked until read size is
> + * equal to requested size.
> + *
> + * Returns: 1 if all bytes were read, 0 if end-of-file
> + *          occurs without data, or -1 on error
> + */
> +int qio_channel_read_peek_all(QIOChannel *ioc,
> +                              void* buf,
> +                              size_t nbytes,
> +                              Error **errp);
> +
>  /**
>   * qio_channel_set_blocking:
>   * @ioc: the channel object
> diff --git a/io/channel-socket.c b/io/channel-socket.c
> index b76dca9cc1..b99f5dfda6 100644
> --- a/io/channel-socket.c
> +++ b/io/channel-socket.c
> @@ -705,6 +705,32 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
>  }
>  #endif /* WIN32 */
>  
> +static ssize_t qio_channel_socket_read_peek(QIOChannel *ioc,
> +                                            void *buf,
> +                                            size_t nbytes,
> +                                            Error **errp)
> +{
> +    QIOChannelSocket *sioc = QIO_CHANNEL_SOCKET(ioc);
> +    ssize_t bytes = 0;
> +
> +retry:
> +    bytes = recv(sioc->fd, buf, nbytes, MSG_PEEK);
> +
> +    if (bytes < 0) {
> +        if (errno == EINTR) {
> +            goto retry;
> +        }
> +        if (errno == EAGAIN) {
> +            return QIO_CHANNEL_ERR_BLOCK;
> +        }
> +
> +        error_setg_errno(errp, errno,
> +                         "Unable to read from peek of socket");
> +        return -1;
> +    }
> +
> +    return bytes;
> +}
>  
>  #ifdef QEMU_MSG_ZEROCOPY
>  static int qio_channel_socket_flush(QIOChannel *ioc,
> @@ -902,6 +928,7 @@ static void qio_channel_socket_class_init(ObjectClass 
> *klass,
>  
>      ioc_klass->io_writev = qio_channel_socket_writev;
>      ioc_klass->io_readv = qio_channel_socket_readv;
> +    ioc_klass->io_read_peek = qio_channel_socket_read_peek;
>      ioc_klass->io_set_blocking = qio_channel_socket_set_blocking;
>      ioc_klass->io_close = qio_channel_socket_close;
>      ioc_klass->io_shutdown = qio_channel_socket_shutdown;
> diff --git a/io/channel.c b/io/channel.c
> index 0640941ac5..a2d9b96f3f 100644
> --- a/io/channel.c
> +++ b/io/channel.c
> @@ -346,6 +346,45 @@ int qio_channel_write_all(QIOChannel *ioc,
>      return qio_channel_writev_all(ioc, &iov, 1, errp);
>  }
>  
> +int qio_channel_read_peek_all(QIOChannel *ioc,
> +                              void* buf,
> +                              size_t nbytes,
> +                              Error **errp)
> +{
> +   QIOChannelClass *klass = QIO_CHANNEL_GET_CLASS(ioc);
> +   ssize_t bytes = 0;
> +
> +   if (!klass->io_read_peek) {
> +       error_setg(errp, "Channel does not support read peek");
> +       return -1;
> +   }

There's no io_read_peek for  QIOChannelTLS, so we'll hit this
error...


> diff --git a/migration/migration.c b/migration/migration.c
> index 739bb683f3..f4b6f278a9 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -733,31 +733,40 @@ void migration_ioc_process_incoming(QIOChannel *ioc, 
> Error **errp)
>  {
>      MigrationIncomingState *mis = migration_incoming_get_current();
>      Error *local_err = NULL;
> -    bool start_migration;
>      QEMUFile *f;
> +    bool default_channel = true;
> +    uint32_t channel_magic = 0;
> +    int ret = 0;
>  
> -    if (!mis->from_src_file) {
> -        /* The first connection (multifd may have multiple) */
> +    if (migrate_use_multifd() && !migration_in_postcopy()) {
> +        ret = qio_channel_read_peek_all(ioc, (void *)&channel_magic,
> +                                        sizeof(channel_magic), &local_err);
> +
> +        if (ret != 1) {
> +            error_propagate(errp, local_err);
> +            return;
> +        }

....and thus this will fail for TLS channels AFAICT.


> +
> +        default_channel = (channel_magic == cpu_to_be32(QEMU_VM_FILE_MAGIC));
> +    } else {
> +        default_channel = !mis->from_src_file;
> +    }

With regards,
Daniel
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