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

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?