Anthony Liguori wrote:
Avi Kivity wrote:
Fine, let's say we did that, it's *still* racy because at time 3,
the guest may hot remove cpu 2 on it's own since the guests VCPUs
get to run in parallel to the monitor.
A guest can't hotremove a vcpu. It may offline a vcpu, but that's
not the same.
Obviously, if both the guest and the management application can
initiate the same action, then there will be races. But I don't
think that's how things should be -- the guest should request a vcpu
to be removed (or added), management thinks and files forms in
triplicate, then hotadds or hotremoves the vcpu (most likely after it
is no longer needed).
With the proper beaurocracy, there is no race.
You still have the same basic problem:
time 0: (qemu) notify-enable vnc-events
time 1: (qemu) foo <no newline>
time 4: <newline>
time 5: notification: client connected
time 0: vnc client connects
time 2: vnc client disconnects
At time 5, when the management app gets the notification, it cannot
make any assumptions about the state of the system. You still need
timestamps.
You don't even need the foo <no newline> to trigger this, qemu->user
traffic can be arbitrarily delayed (I don't think we should hold
notifications on partial input anyway). But there's no race here.
The notification at time 5 means that the connect happened sometime
before time 5, and that it may not be true now. The user cannot assume
anything. A race can only happen against something the user initiated.
Suppose we're implementing some kind of single sign on:
(qemu) notify vnc on
... time passes, we want to allow members of group x to log in
(qemu) vnc_set_acl group:x
OK
(qemu)
notification: vnc connect aliguori
(qemu)
with a single monitor, we can be sure that the connect happened the
vnc_set_acl. If the notification arrives on a different session, we
have no way of knowing that.
And even if you somehow eliminate the issue around masking
notifications, you still have socket buffering that introduces the
same problem.
If you have one monitor, the problem is much simpler, since events
travelling in the same direction (command acknowledge and a
notification) cannot be reordered. With a command+wait, the problem
is inherent.
Command acknowledge is not an event. Events are out-of-band. Command
completions are in-band. Right now, they are synchronous and
That's all correct, but I don't see how that changes anything.
I expect that in the short term future, we'll have a non-human monitor
mode that allows commands to be asynchronous.
Then let's defer this until then? 'wait' is not useful for humans, they
won't be retyping 'wait' every time something happens.
However, it's a mistake to muddle the distinction between an in-band
completion and an out-of-band event. You cannot relate the
out-of-band events commands.
I can, if one happens before the other, and I have a single stream of
command completions and event notifications.
The best you can do is stick a time stamp on a notification and make
sure the management tool understands that the notification is
reflectively of the state when the event happened, not of the
current state.
Timestamps are really bad. They don't work at all if the management
application is not on the same host. They work badly if it is on the
same host, since commands and events will be timestamped at different
processes.
Timestamps are relative, not absolutely. They should not be used to
associate anything with the outside world. In fact, I have no problem
making the timestamps relative to QEMU startup just to ensure that
noone tries to do something silly like associate notification
timestamps with system time.
Dunno, seems totally artificial to me to have to introduce timestamps to
compensate for different delays in multiple sockets that we introduced
five patches earlier.
Please, let's keep this simple.
FWIW, this problem is not at all unique to QEMU and is generally
true of most protocols that support an out-of-band notification
mechanism.
command+wait makes it worse. Let's stick with established practice.
What's the established practice? Do you know of any protocol that is
line based that does notifications like this?
I guess most MUDs?
IMAP IDLE is pretty close to "wait-forever".
IMAP IDLE can be terminated by the client, and so does not require
multiple sessions (though IMAP supports them).
--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.
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