[libvirt] [Qemu-devel] [PATCH v7 6/6] block: Enable qemu_open/close to work with fd sets
Corey Bryant
coreyb at linux.vnet.ibm.com
Wed Aug 8 13:54:20 UTC 2012
On 08/08/2012 09:02 AM, Stefan Hajnoczi wrote:
> On Tue, Aug 07, 2012 at 11:58:28AM -0400, Corey Bryant wrote:
>> @@ -2566,6 +2567,92 @@ FdsetInfoList *qmp_query_fdsets(Error **errp)
>> return fdset_list;
>> }
>>
>> +int monitor_fdset_get_fd(int64_t fdset_id, int flags)
>> +{
>> + mon_fdset_t *mon_fdset;
>> + mon_fdset_fd_t *mon_fdset_fd;
>> + int mon_fd_flags;
>> +
>> + QLIST_FOREACH(mon_fdset, &mon_fdsets, next) {
>> + if (mon_fdset->id != fdset_id) {
>> + continue;
>> + }
>> + QLIST_FOREACH(mon_fdset_fd, &mon_fdset->fds, next) {
>> + if (mon_fdset_fd->removed) {
>> + continue;
>> + }
>
> This makes me wonder about immediately closing in remove-fd and drop the
> removed field. This way, code using mon_fdset->fds does not need to
> worry about removed=true fds.
>
>
The reason we don't immediately close in remove-fd is so that the client
doesn't have to keep track of what fd's are in use by QEMU.
Let's say libvirt uses add-fd to add fd=4 (O_RDONLY) and fd=5 (O_RDWR)
to fd set 2 and they both refer to /mnt/nfs/data.img. libvirt can then
issue a command that uses the fd (e.g. drive_add file=/dev/fdsets/2).
QEMU then opens and closes that file as it desires by dup'ing the fd in
the fd set that has matching access mode (O_RDONLY or O_RDWR) and
closing the dup'd fd.
By not closing the fd immediately in remove-fd, we allow the client to
issue a command like drive_open and immediately follow it with a
remove-fd and not have to worry about determining when QEMU is done
using the fd.
--
Regards,
Corey
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