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

[Yury Kotov]

15.04.2019, 13:25, "Daniel P. Berrangé" <address@hidden>:
> 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.
>

Oh, sorry. This use case is not obvious. We used add-fd to pass fd for
migrate-incoming and such way has described problems.

Regards,
Yury