Re: [RFC PATCH v3 3/3] migration: Add fd to FileMigrationArgs

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

On Fri, Mar 15, 2024 at 04:54:27PM -0400, Peter Xu wrote:
> On Fri, Mar 15, 2024 at 03:01:09PM -0300, Fabiano Rosas wrote:
> > Peter Xu <peterx@redhat.com> writes:
> > 
> > > [I queued patch 1-2 into -stable, leaving this patch for further
> > >  discussions]
> > >
> > > On Fri, Mar 15, 2024 at 08:55:42AM +0000, Daniel P. Berrangé wrote:
> > >> The 'file:' protocol eventually calls into qemu_open, and this
> > >> transparently allows for FD passing using /dev/fdset/NNN syntax
> > >> to pass in FDs. 
> > >
> > > If it always use /dev/fdsets for files, does it mean that the newly added
> > > SOCKET_ADDRESS_TYPE_FD support on mapped-ram will never be used (so we can
> > > drop them)?
> > 
> > We already have SOCKET_ADDRESS_TYPE_FD + file since 8.2 when the
> > MigrationAddress was added. So this:
> > 
> > 'channels': [ { 'channel-type': 'main',
> >                 'addr': { 'transport': 'socket',
> >                 'type': 'fd',
> >                 'str': 'fdname' } } ]
> > 
> > works without multifd and without mapped-ram if the fd is a file or
> > socket.
> > 
> > So yes, you're correct, but given we already have this^ it would be
> > perhaps more confusing for users to allow it, but not allow the very
> > same JSON when multifd=true, mapped-ram=true.
> 
> I don't think the fd: protocol (no matter the old "fd:", or the new JSON
> format) is trivial to use. If libvirt didn't use it I won't be surprised to
> see nobody using it.  I want us to avoid working on things that nobody is
> using, or has a better replacement.
> 
> So even if Libvirt supports both, I'm wondering whether /dev/fdset/ works
> for all the cases that libvirt needs.  I am aware that the old getfd has
> the monitor limitation so that if the QMP disconnected and reconnect, the
> fd can be gone.  However I'm not sure whether that's the only reason to
> have add-fd, and also not sure whether it means add-fd is always preferred,
> so that maybe we can consider obsolete getfd?

Historically libvirt primariily uses the 'fd:' protocol, with a
socket FD. It never gives QEMU a plain file FD, since it has
always added its "iohelper" as a MITM, in order to add O_DIRECT
on top.

The 'getfd' command is something that is needed when talking to
QEMU for any API that involves a "SocketAddress" QAPI type,
which is applicable for migration.

With the introduction of 'MigrationAddress', the 'socket' protocol
is backed by 'SocketAddress' and thus supports FD passing for
sockets (or potentally pipes too), in combination with 'getfd'.

With the 'file' protocol in 'MigrationAddress', since it gets
backed by qemu_open(), then /dev/fdset/NN and 'add-fd' provide
passing for plain files.



With regards,
Daniel
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