On 02/24/2011 02:54 AM, Avi Kivity wrote:
On 02/23/2011 10:18 PM, Anthony Liguori wrote:
Then the management stack has to worry about yet another way of
interacting via qemu.
{ 'StateItem': { 'key': 'str', 'value': 'str' } }
{ 'StateSection': { 'kind': 'str', 'name': 'str', 'items': [
'StateItem' ] } }
{ 'StateInfo': { 'sections': [ 'StateSection' ] } }
{ 'query-state', {}, {}, 'StateInfo' }
A management tool never need to worry about anything other than this
command if it so chooses. If we have the pre-machine init mode for
0.16, then this can even be used to inspect state without running a
guest.
So we have yet another information tree. If we store the cd-rom eject
state here, then we need to make an association between the device
path of the cd-rom, and the StateItem key.
And this linkage is key.
Let's say I launch QEMU with:
qemu -cdrom ~/foo.img
And then in the monitor, I do:
(qemu) eject ide1-cd0
The question is, what command can I now use to launch the same qemu
instance?
When I think of stateful config, what I really think of is a way to spit
out a command line that essentially becomes, "this is how you now launch
QEMU".
In this case, it would be:
qemu -cdrom ~/foo.img -device ide-disk,id=ide1-cd0,drive=
Or, we could think of this in terms of:
qemu -cdrom ~/foo.img -readconfig foo.cfg
Where foo.cfg contained:
[device "ide1-cd0"]
driver="ide-disk"
drive=""
So what I'm really suggesting is that we generate foo.cfg whenever
monitor commands do things that change the command line and introduce a
new option to reflect this, IOW:
qemu -cdrom ~/foo.img -config foo.cfg
Far better to store it in the device itself. For example, we could
make a layered block format driver that stores the eject state and a
"backing file" containing the actual media. Eject and media change
would be recorded in the block format driver's state. You could then
hot-unplug a USB cd-writer and hot-plug it back into a different
guest, implementing a virtual sneakernet.
I think you're far too hung up on "store it in the device itself". The
recipe to create the device model is not intrinsic to the device model.
It's an independent thing that's a combination of the command line
arguments and any executed monitor commands.
Maybe a better way to think about the stateful config file is a
mechanism to replay the monitor history.
The fact that the state is visible in the filesystem is an
implementation detail.
A detail that has to be catered for by the management stack - it has
to provide a safe place for it, back it up, etc.
If it cares for QEMU to preserve state. Today, this all gets thrown away.
It doesn't work for eject unless you interpose an acknowledged
event. Ultimately, this is a simple problem. If you want
reliability, we either need symmetric RPCs so that the device model
can call (and wait) to the management layer to acknowledge a change
or QEMU can post an event to the management layer, and maintain the
state in a reliable fashion.
I don't see why it doesn't work. Please explain.
1) guest eject
2) qemu posts eject event
3) qemu acknowledges eject to the guest
4) management tool sees eject event and updates guest config
There's a race between 3 & 4. It can only be addressed by interposing 4
between 2 and 3 OR making qemu persist this state between 2 and 3 such
that the management tool can reliably query it.
You still have the race condition around guest initiated events
like eject. Unless you have an acknowledged event from a
management tool (which we can't do in QMP today) whereas you don't
complete the guest initiated eject operation until management ack's
it, we need to store that state ourself.
I don't see why.
If management crashes, it queries the eject state when it reconnects
to qemu.
If qemu crashes, the eject state is lost, but that is fine. My
CD-ROM drive tray pulls itself in when the machine is started.
Pick any of a number of possible events that change the machine's
state. We can wave our hands at some things saying they don't matter
and do one off solutions for others, or we can just have a robust way
of handling this consistently.
Both block live copy and cd-rom eject state can be solved with layered
block format drivers. I don't think a central place for random data
makes sense. State belongs near the device that maintains it, esp. if
the device is hot-pluggable, so it's easy to associate the state with
the device.
You're introducing the need for additional code in the management
layer, the care and feeding for the stateful non-config file.
If a management layer ignores the stateful non-config file, as you
like to call it, it'll get the same semantics it has today. I think
managing a single thing is a whole lot easier than managing an NVRAM
file, a block migration layering file, and all of the future things
we're going to add once we decide they are important too.
I disagree. Storing NVRAM as a disk image is a simple extension of
existing management tools. Block live-copy and cd-rom eject state
also make sense as per-image state if you take hotunplug and hotplug
into account.
Everything can be stored in a block driver but when the data is highly
structured, isn't it nice to expose it in a structured, human readable
way? I know I'd personally prefer a text representation of CMOS than a
binary blob.
If qemu crashes, these events are meaningless. If management
crashes, it has to query qemu for all state that it wants to keep
track of via events.
Think power failure, not qemu crash. In the event of a power
failure, any hardware change initiated by the guest ought to be
consistent with when the guest has restarted. If you eject the
CDROM tray and then lose power, its still ejected after the power
comes back on.
Not on all machines.
Let's list guest state which is independent of power. That would be
wither NVRAM of various types, or physical alterations. CD-ROM
eject is one. Are there others?
Any indirect qemu state. Block migration is an example, but other
examples would be VNC server information (like current password), WCE
setting (depending on whether we modelled eeprom for the drivers),
and persisted device settings (lots of devices have eeprom these days).
Device settings should be stored with the devices, not with qemu.
Suppose we take the cold-plug on startup via the monitor approach. So
we start with a bare machine, cold plug stuff into it. Now qemu has
to reconcile the stateful non-config file with the hardware. What if
something has changed? A device moved into a different slot?
Sorry, I'm confused. Is there anything in the stateful config file when
we start up? If so, the act of starting up will add a bunch of hardware.
If a network card has eeprom, we can specify it with -device
rtl8139,eeprom=id, where id specifies a disk image for the eeprom.
We could, but then we'll end up with a bunch of little block devices.
That seems less than ideal to me.
Technically, mac address is stored on eeprom and we store that as a
device property today. We can't persist device properties even though
you can change the mac address of a network card and it does persist
across reboots. Are you advocating that we introduce an eeprom for
every network card (all in a slightly different format) and have special
tools to manipulate the eeprom to store and view the mac address?
Regards,
Anthony Liguori
I think my solution (multiplexing block format driver) fits the
requirements for live-copy perfectly. In fact it has a name - it's
a RAID-1 driver started in degraded mode. It could be useful other
use cases.
It feels a bit awkward to me to be honest.
Not to me.