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Re: [libvirt] [Qemu-devel] [PATCH v7 6/6] block: Enable qemu_open/close to work with fd sets





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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