Re: [PATCH v2] migration: check magic value for deciding the mapping of channels

["manish.mishra" <manish.mishra@nutanix.com>]

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Hi Everyone, Just a gentle reminder for review. :)

Thanks

Manish Mishra

On 07/11/22 10:21 pm, 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 main 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, tls live
> migrations already does tls handshake before creating other channels, so
> this issue is not possible with tls, hence this logic is avoided for tls
> live migrations. This patch uses MSG_PEEK to check the magic number of
> channels so that current data/control stream management remains
> un-effected.
>
> Suggested-by: Daniel P. Berrangé<berrange@redhat.com>
> Signed-off-by: manish.mishra<manish.mishra@nutanix.com>
>
> v2:
>    TLS does not support MSG_PEEK, so V1 was broken for tls live
>    migrations. For tls live migration, while initializing main channel
>    tls handshake is done before we can create other channels, so this
>    issue is not possible for tls live migrations. In V2 added a check
>    to avoid checking magic number for tls live migration and fallback
>    to older method to decide mapping of channels on destination side.
> ---
>   include/io/channel.h     | 25 +++++++++++++++++++++++
>   io/channel-socket.c      | 27 ++++++++++++++++++++++++
>   io/channel.c             | 39 +++++++++++++++++++++++++++++++++++
>   migration/migration.c    | 44 +++++++++++++++++++++++++++++-----------
>   migration/multifd.c      | 12 ++++-------
>   migration/multifd.h      |  2 +-
>   migration/postcopy-ram.c |  5 +----
>   migration/postcopy-ram.h |  2 +-
>   8 files changed, 130 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;
> +   }
> +
> +   while (bytes < nbytes) {
> +       bytes = klass->io_read_peek(ioc,
> +                                   buf,
> +                                   nbytes,
> +                                   errp);
> +
> +       if (bytes == QIO_CHANNEL_ERR_BLOCK) {
> +            if (qemu_in_coroutine()) {
> +                qio_channel_yield(ioc, G_IO_OUT);
> +            } else {
> +                qio_channel_wait(ioc, G_IO_OUT);
> +            }
> +            continue;
> +       }
> +       if (bytes == 0) {
> +           error_setg(errp,
> +                      "Unexpected end-of-file on channel");
> +           return 0;
> +       }
> +       if (bytes < 0) {
> +           return -1;
> +       }
> +   }
> +
> +   return 1;
> +}
>   
>   int qio_channel_set_blocking(QIOChannel *ioc,
>                                 bool enabled,
> diff --git a/migration/migration.c b/migration/migration.c
> index 739bb683f3..406a9e2f72 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -733,31 +733,51 @@ 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() &&
> +        !migrate_use_tls()) {
> +        /*
> +         * With multiple channels, it is possible that we receive channels
> +         * out of order on destination side, causing incorrect mapping of
> +         * source channels on destination side. Check channel MAGIC to
> +         * decide type of channel. Please note this is best effort, postcopy
> +         * preempt channel does not send any magic number so avoid it for
> +         * postcopy live migration. Also tls live migration already does
> +         * tls handshake while initializing main channel so with tls this
> +         * issue is not possible.
> +         */
> +        ret = qio_channel_read_peek_all(ioc, (void *)&channel_magic,
> +                                        sizeof(channel_magic), &local_err);
> +
> +        if (ret != 1) {
> +            error_propagate(errp, local_err);
> +            return;
> +        }
> +
> +        default_channel = (channel_magic == cpu_to_be32(QEMU_VM_FILE_MAGIC));
> +    } else {
> +        default_channel = !mis->from_src_file;
> +    }
> +
> +    if (default_channel) {
>           f = qemu_file_new_input(ioc);
>   
>           if (!migration_incoming_setup(f, errp)) {
>               return;
>           }
> -
> -        /*
> -         * Common migration only needs one channel, so we can start
> -         * right now.  Some features need more than one channel, we wait.
> -         */
> -        start_migration = !migration_needs_multiple_sockets();
>       } else {
>           /* Multiple connections */
>           assert(migration_needs_multiple_sockets());
>           if (migrate_use_multifd()) {
> -            start_migration = multifd_recv_new_channel(ioc, &local_err);
> +            multifd_recv_new_channel(ioc, &local_err);
>           } else {
>               assert(migrate_postcopy_preempt());
>               f = qemu_file_new_input(ioc);
> -            start_migration = postcopy_preempt_new_channel(mis, f);
> +            postcopy_preempt_new_channel(mis, f);
>           }
>           if (local_err) {
>               error_propagate(errp, local_err);
> @@ -765,7 +785,7 @@ void migration_ioc_process_incoming(QIOChannel *ioc, Error **errp)
>           }
>       }
>   
> -    if (start_migration) {
> +    if (migration_has_all_channels()) {
>           /* If it's a recovery, we're done */
>           if (postcopy_try_recover()) {
>               return;
> diff --git a/migration/multifd.c b/migration/multifd.c
> index 586ddc9d65..be86a4d07f 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -1220,11 +1220,9 @@ bool multifd_recv_all_channels_created(void)
>   
>   /*
>    * Try to receive all multifd channels to get ready for the migration.
> - * - Return true and do not set @errp when correctly receiving all channels;
> - * - Return false and do not set @errp when correctly receiving the current one;
> - * - Return false and set @errp when failing to receive the current channel.
> + * Sets @errp when failing to receive the current channel.
>    */
> -bool multifd_recv_new_channel(QIOChannel *ioc, Error **errp)
> +void multifd_recv_new_channel(QIOChannel *ioc, Error **errp)
>   {
>       MultiFDRecvParams *p;
>       Error *local_err = NULL;
> @@ -1237,7 +1235,7 @@ bool multifd_recv_new_channel(QIOChannel *ioc, Error **errp)
>                                   "failed to receive packet"
>                                   " via multifd channel %d: ",
>                                   qatomic_read(&multifd_recv_state->count));
> -        return false;
> +        return;
>       }
>       trace_multifd_recv_new_channel(id);
>   
> @@ -1247,7 +1245,7 @@ bool multifd_recv_new_channel(QIOChannel *ioc, Error **errp)
>                      id);
>           multifd_recv_terminate_threads(local_err);
>           error_propagate(errp, local_err);
> -        return false;
> +        return;
>       }
>       p->c = ioc;
>       object_ref(OBJECT(ioc));
> @@ -1258,6 +1256,4 @@ bool multifd_recv_new_channel(QIOChannel *ioc, Error **errp)
>       qemu_thread_create(&p->thread, p->name, multifd_recv_thread, p,
>                          QEMU_THREAD_JOINABLE);
>       qatomic_inc(&multifd_recv_state->count);
> -    return qatomic_read(&multifd_recv_state->count) ==
> -           migrate_multifd_channels();
>   }
> diff --git a/migration/multifd.h b/migration/multifd.h
> index 519f498643..913e4ba274 100644
> --- a/migration/multifd.h
> +++ b/migration/multifd.h
> @@ -18,7 +18,7 @@ void multifd_save_cleanup(void);
>   int multifd_load_setup(Error **errp);
>   int multifd_load_cleanup(Error **errp);
>   bool multifd_recv_all_channels_created(void);
> -bool multifd_recv_new_channel(QIOChannel *ioc, Error **errp);
> +void multifd_recv_new_channel(QIOChannel *ioc, Error **errp);
>   void multifd_recv_sync_main(void);
>   int multifd_send_sync_main(QEMUFile *f);
>   int multifd_queue_page(QEMUFile *f, RAMBlock *block, ram_addr_t offset);
> diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> index b9a37ef255..f84f783ab4 100644
> --- a/migration/postcopy-ram.c
> +++ b/migration/postcopy-ram.c
> @@ -1539,7 +1539,7 @@ void postcopy_unregister_shared_ufd(struct PostCopyFD *pcfd)
>       }
>   }
>   
> -bool postcopy_preempt_new_channel(MigrationIncomingState *mis, QEMUFile *file)
> +void postcopy_preempt_new_channel(MigrationIncomingState *mis, QEMUFile *file)
>   {
>       /*
>        * The new loading channel has its own threads, so it needs to be
> @@ -1548,9 +1548,6 @@ bool postcopy_preempt_new_channel(MigrationIncomingState *mis, QEMUFile *file)
>       qemu_file_set_blocking(file, true);
>       mis->postcopy_qemufile_dst = file;
>       trace_postcopy_preempt_new_channel();
> -
> -    /* Start the migration immediately */
> -    return true;
>   }
>   
>   /*
> diff --git a/migration/postcopy-ram.h b/migration/postcopy-ram.h
> index 6147bf7d1d..25881c4127 100644
> --- a/migration/postcopy-ram.h
> +++ b/migration/postcopy-ram.h
> @@ -190,7 +190,7 @@ enum PostcopyChannels {
>       RAM_CHANNEL_MAX,
>   };
>   
> -bool postcopy_preempt_new_channel(MigrationIncomingState *mis, QEMUFile *file);
> +void postcopy_preempt_new_channel(MigrationIncomingState *mis, QEMUFile *file);
>   int postcopy_preempt_setup(MigrationState *s, Error **errp);
>   int postcopy_preempt_wait_channel(MigrationState *s);
>   

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