Re: [Qemu-devel] [PATCH] migration: Fix handling fd protocol

[Daniel P . Berrangé]

On Mon, Apr 15, 2019 at 01:17:06PM +0300, Yury Kotov wrote:
> 15.04.2019, 13:11, "Daniel P. Berrangé" <address@hidden>:
> > On Mon, Apr 15, 2019 at 12:50:08PM +0300, Yury Kotov wrote:
> >>  Hi,
> >>
> >>  Just to clarify. I see two possible solutions:
> >>
> >>  1) Since the migration code doesn't receive fd, it isn't responsible for
> >>  closing it. So, it may be better to use migrate_fd_param for both
> >>  incoming/outgoing and add dupping for migrate_fd_param. Thus, clients must
> >>  close the fd themselves. But existing clients will have a leak.
> >
> > We can't break existing clients in this way as they are correctly
> > using the monitor with its current semantics.
> >
> >>  2) If we don't duplicate fd, then at least we should remove fd from
> >>  the corresponding list. Therefore, the solution is to fix qemu_close to 
> >> find
> >>  the list and remove fd from it. But qemu_close is currently consistent 
> >> with
> >>  qemu_open (which opens/dups fd), so adding additional logic might not be
> >>  a very good idea.
> >
> > qemu_close is not appropriate place to deal with something speciifc
> > to the montor.
> >
> >>  I don't see any other solution, but I might miss something.
> >>  What do you think?
> >
> > All callers of monitor_get_fd() will close() the FD they get back.
> > Thus monitor_get_fd() should remove it from the list when it returns
> > it, and we should add API docs to monitor_get_fd() to explain this.
> >
> Ok, it sounds reasonable. But monitor_get_fd is only about outgoing migration.
> But what about the incoming migration? It doesn't use monitor_get_fd but just
> converts input string to int and use it as fd.

The incoming migration expects the FD to be passed into QEMU by the mgmt
app when it is exec'ing the QEMU binary. It doesn't interact with the
monitor at all AFAIR.

Regards,
Daniel
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