On Fri, Mar 15, 2024 at 12:20:40AM -0300, Fabiano Rosas wrote: > The fd: URI has supported migration to a file or socket since before > QEMU 8.2. In 8.2 we added the file: URI that supported migration to a > file. So now we have two ways (three if you count exec:>cat) to > migrate to a file. Fine. > > However, > > In 8.2 we also added the new qmp_migrate API that uses a JSON channel > list instead of the URI. It added two migration transports SOCKET and > FILE. It was decided that the new API would classify the fd migration > as a type of socket migration, neglecting the fact that the fd.c code > also supported file migrations. > > In 9.0 we're adding support for fd + multifd + mapped-ram, which is > tied to the file migration. This was implemented in fd.c, which is > only reachable when the SOCKET address type is used. > > The result of this is that we're asking users of the new API to create (1) > something called a "socket" to perform migration to a plain file. And > creating something called a "file" provides no way of passing in a > file descriptor. This is confusing.
The 'file:' protocol eventually calls into qemu_open, and this transparently allows for FD passing using /dev/fdset/NNN syntax to pass in FDs. > diff --git a/qapi/migration.json b/qapi/migration.json > index aa1b39bce1..37f4b9c6fb 100644 > --- a/qapi/migration.json > +++ b/qapi/migration.json > @@ -1656,13 +1656,20 @@ > # > # @filename: The file to receive the migration stream > # > +# @fd: A file descriptor name or number. File descriptors must be > +# first added with the 'getfd' command. (since 9.0). > +# > # @offset: The file offset where the migration stream will start > # > +# Since 9.0, all members are optional, but at least one of @filename > +# or @fd are required. > +# > # Since: 8.2 > ## > { 'struct': 'FileMigrationArgs', > - 'data': { 'filename': 'str', > - 'offset': 'uint64' } } > + 'data': { '*filename': 'str', > + '*fd': 'str', > + '*offset': 'uint64' } } Adding 'fd' here is not desirable, because 'filename' is resolved via qemu_open which allows for FD passing without introducing any new syntax in interfaces which take filenames. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|