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

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

On Tue, Nov 15, 2022 at 06:11:30PM +0000, Daniel P. Berrangé wrote:
> 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.

Actually ignore that.  We should not add any new APIs at
all.  Instead the io_readv callback, and the qio_channel_read*all()
methods should gain a 'int flags' parameter, in the same way that
the write methods have one. Then there should be as
QIO_CHANNEL_READ_FLAG_PEEK constant defined.


With regards,
Daniel
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