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

[Kevin Wolf]

Am 06.08.2012 15:32, schrieb Corey Bryant:
> 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.

Hm, I see.

>> 2. Save a monitor reference with the fdset information.
>>
> 
> Are you saying that each monitor would have the same copy of fdset 
> information?

This one doesn't really make sense indeed...

> 
>> 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?

Would get you some kind of consistency at least, even though I don't
like that secondary monitors can't use the functionality.

Can't we make the fdset information global, so that a qemu_open/close()
searches all of them, but let it have a Monitor* owner for keeping track
whether it's in use?

Kevin