Re: [PATCH v2 19/29] migration/multifd: Add outgoing QIOChannelFile support

[Fabiano Rosas]

Daniel P. Berrangé <berrange@redhat.com> writes:

> On Wed, Oct 25, 2023 at 11:12:38AM -0300, Fabiano Rosas wrote:
>> Daniel P. Berrangé <berrange@redhat.com> writes:
>> 
>> > On Mon, Oct 23, 2023 at 05:35:58PM -0300, Fabiano Rosas wrote:
>> >> Allow multifd to open file-backed channels. This will be used when
>> >> enabling the fixed-ram migration stream format which expects a
>> >> seekable transport.
>> >> 
>> >> The QIOChannel read and write methods will use the preadv/pwritev
>> >> versions which don't update the file offset at each call so we can
>> >> reuse the fd without re-opening for every channel.
>> >> 
>> >> Note that this is just setup code and multifd cannot yet make use of
>> >> the file channels.
>> >> 
>> >> Signed-off-by: Fabiano Rosas <farosas@suse.de>
>> >> ---
>> >>  migration/file.c      | 64 +++++++++++++++++++++++++++++++++++++++++--
>> >>  migration/file.h      | 10 +++++--
>> >>  migration/migration.c |  2 +-
>> >>  migration/multifd.c   | 14 ++++++++--
>> >>  migration/options.c   |  7 +++++
>> >>  migration/options.h   |  1 +
>> >>  6 files changed, 90 insertions(+), 8 deletions(-)
>> >> 
>> >> diff --git a/migration/file.c b/migration/file.c
>> >> index cf5b1bf365..93b9b7bf5d 100644
>> >> --- a/migration/file.c
>> >> +++ b/migration/file.c
>> >> @@ -17,6 +17,12 @@
>> >
>> >> +void file_send_channel_create(QIOTaskFunc f, void *data)
>> >> +{
>> >> +    QIOChannelFile *ioc;
>> >> +    QIOTask *task;
>> >> +    Error *errp = NULL;
>> >> +
>> >> +    ioc = qio_channel_file_new_path(outgoing_args.fname,
>> >> +                                    outgoing_args.flags,
>> >> +                                    outgoing_args.mode, &errp);
>> >> +    if (!ioc) {
>> >> +        file_migration_cancel(errp);
>> >> +        return;
>> >> +    }
>> >> +
>> >> +    task = qio_task_new(OBJECT(ioc), f, (gpointer)data, NULL);
>> >> +    qio_task_run_in_thread(task, qio_channel_file_connect_worker,
>> >> +                           (gpointer)data, NULL, NULL);
>> >> +}
>> >> +
>> >>  void file_start_outgoing_migration(MigrationState *s, const char 
>> >> *filespec,
>> >>                                     Error **errp)
>> >>  {
>> >> -    g_autofree char *filename = g_strdup(filespec);
>> >>      g_autoptr(QIOChannelFile) fioc = NULL;
>> >> +    g_autofree char *filename = g_strdup(filespec);
>> >>      uint64_t offset = 0;
>> >>      QIOChannel *ioc;
>> >> +    int flags = O_CREAT | O_TRUNC | O_WRONLY;
>> >> +    mode_t mode = 0660;
>> >>  
>> >>      trace_migration_file_outgoing(filename);
>> >>  
>> >> @@ -50,12 +105,15 @@ void file_start_outgoing_migration(MigrationState 
>> >> *s, const char *filespec,
>> >>          return;
>> >>      }
>> >>  
>> >> -    fioc = qio_channel_file_new_path(filename, O_CREAT | O_WRONLY | 
>> >> O_TRUNC,
>> >> -                                     0600, errp);
>> 
>> By the way, we're experimenting with add-fd to flesh out the interface
>> with libvirt and I see that the flags here can conflict with the flags
>> set on the fd passed through `virsh --pass-fd ...` due to this at
>> monitor_fdset_dup_fd_add():
>> 
>>     if ((flags & O_ACCMODE) == (mon_fd_flags & O_ACCMODE)) {
>>         fd = mon_fdset_fd->fd;
>>         break;
>>     }
>> 
>> We're requiring the O_RDONLY, O_WRONLY, O_RDWR flags defined here to
>> match the fdset passed into QEMU. Should we just sync the code of the
>> two projects to use the same flags? That feels a little clumsy to me.
>
> Is there a reason for libvirt to have set O_RDONLY for a file used
> for outgoing migration ?  I can't recall off-hand.
>

The flags need to match exactly, so either libvirt or QEMU could in the
future decide to use O_RDWR. Then we'd have a compatibility problem when
passing the fds around.