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

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

On Wed, Nov 16, 2022 at 04:49:18PM +0530, manish.mishra wrote:
> 
> On 16/11/22 12:20 am, Daniel P. Berrangé wrote:
> > 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.
> 
> Hi Daniel, As MSG_PEEK always reads from top even if there were
> previos partial reads, so current |qio_channel_readv_all_eofmay
> not work? I can keep things upto ||qio_channel_readv as common
> for both with flags parameters but have separate
> ||qio_channel_readv_peek_all_eof? Does something like that works.|||

Simplest is probably to just not add 'flags' to the 'all' variants,
just the non-'all' varants.

With regards,
Daniel
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