Re: [Qemu-devel] [PATCH v5 6/6] block: Enable qemu_open/close to work with fd sets

[Corey Bryant <coreyb@linux.vnet.ibm.com>]

On 08/06/2012 05:15 AM, Kevin Wolf wrote:
> Am 03.08.2012 00:21, schrieb Corey Bryant:
>>>> @@ -84,6 +158,36 @@ int qemu_open(const char *name, int flags, ...)
>>>>        int ret;
>>>>        int mode = 0;
>>>>
>>>> +#ifndef _WIN32
>>>> +    const char *fdset_id_str;
>>>> +
>>>> +    /* Attempt dup of fd from fd set */
>>>> +    if (strstart(name, "/dev/fdset/", &fdset_id_str)) {
>>>> +        int64_t fdset_id;
>>>> +        int fd, dupfd;
>>>> +
>>>> +        fdset_id = qemu_parse_fdset(fdset_id_str);
>>>> +        if (fdset_id == -1) {
>>>> +            errno = EINVAL;
>>>> +            return -1;
>>>> +        }
>>>> +
>>>> +        fd = monitor_fdset_get_fd(default_mon, fdset_id, flags);
>>>
>>> I know that use of default_mon in this patch is not correct, but I
>>> wanted to get these patches out for review. I used default_mon for
>>> testing because cur_mon wasn't pointing to the monitor I'd added fd sets
>>> to.  I need to figure out why.
>>>
>>
>> Does it make sense to use default_mon here?  After digging into this
>> some more, I'm thinking it makes sense, and I'll explain why.
>>
>> It looks like cur_mon can't be used.  cur_mon will point to the monitor
>> object for the duration of a command, and be reset to old_mon (NULL in
>> my case) after the command completes.
>>
>> qemu_open() and qemu_close() are frequently called long after a monitor
>> command has come and gone, so cur_mon won't work.  For example,
>> drive_add will cause qemu_open() to be called, but after the command has
>> completed, the file will keep getting opened/closed during normal QEMU
>> operation.  I'm not sure why, I've just noticed this behavior.
>>
>> Does anyone have any thoughts on this?  It would require fd sets to be
>> added to the default monitor only.
>
> I think we have two design options that would make sense:
>
> 1. Make the file descriptors global instead of per-monitor. Is there a
>     reason why each monitor has its own set of fds? (Also I'm wondering
>     if they survive a monitor disconnect this way?)

I'd prefer to have them associated with a monitor object so that we can 
more easily keep track of whether or not they're in use by a monitor 
connection.

>
> 2. Save a monitor reference with the fdset information.
>

Are you saying that each monitor would have the same copy of fdset 
information?

> Allowing to send file descriptors on every monitor, but making only
> those of the default monitor actually usable, sounds like a bad choice
> to me.

What if we also allow them to be added only to the default monitor?

-- 
Regards,
Corey