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

[Daniel P . Berrangé]

On Mon, Nov 07, 2022 at 04:51:59PM +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 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(-)

This should be two commits, because the 'io' and 'migration'
code are two separate subsystems in QEMU.

> 
> 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);

This API should be called "io_read_peekv" and use
"const struct iovec *iov", such that is matches the
design of 'io_readv'.

There should also be a QIOChannelFeature flag
registered to indicate whether a given channel
impl supports peeking at data.


> @@ -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);

There should be qio_channel_read_peek, qio_channel_read_peekv,
qio_channel_read_peek_all and qio_channel_read_peekv_all.


With regards,
Daniel
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