Re: [Qemu-devel] [PATCH] runstate: do not discard runstate changes when paused

[Jan Kiszka <jan.kiszka@web.de>]

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On 2011-10-05 18:37, Avi Kivity wrote:
> On 10/05/2011 06:31 PM, Jan Kiszka wrote:
>> >>
>> >
>> >  vm_start() should be symmetric with vm_stop().  That is, if a piece of
>> >  code wants to execute with vcpus stopped, it should just run inside a
>> >  stop/start pair.
>> >
>> >  The only confusion can come from the user, if he sees multiple stop
>> >  events and expects that just one cont will continue the vm.  For the
>> >  machine monitor, we should just document that the you have to issue
>> one
>> >  cont for every stop event you see (plus any stops you issue).  It's
>> not
>> >  unnatural - the code that handles a stop_due_to_enospace can work
>> to fix
>> >  the error and issue a cont, disregarding any other stops in progress
>> >  (due to a user pressing the stop button, or migration, or cpu hotplug,
>> >  or whatever).  For the human monitor, it's not so intuitive, but the
>> >  situation is so rare we can just rely on the user to issue cont again.
>>
>> Making this kind of user-visible change would be a bad idea.
> 
> The current situation is a bad idea.
> 
> Consider a user-initiated or qemu-initiated stop; the user starts to
> deal with it, types 'cont', and as the Enter key is being depressed
> another qemu-initiated stop comes along.  The 'cont' restarts the guest
> even though the second event was not dealt with.

You always have this kind of problems when you attach two keyboards to
the same console. A counting stop/cont will just create different
effects of the same problem but not solve it.

> 
>> We are talking about multiple stop states here, but only a single
>> function (vm_stop) to enter them - maybe that's not optimal. But the
>> point is that we were missing one stop-to-stop transition. And that
>> needs to be fixed, either inside vm_stop or when it is called.
> 
> Those stops are orthogonal.  There is no relationship between a
> migration stop, a user stop, an ENOSPC stop, a hotplug stop, and a
> debugger stop.  There is no reason to start inventing stop-to-stop
> transitions between them.  A 'cont' intended for one should not undo
> another.

No, they aren't. They are all different states, and we need to model
transitions between them. If there are redundant states, lets collapse them.

> 
> There are two ways to do this, one is to store a set of stop reasons and
> let both 'stop' and 'cont' specify the reason, the other, which is
> simpler but less safe, is to use a reference counting approach.
> 
>>
>> If you want to lock the VM into paused state, add a new symmetric API
>> that does precisely this. That API would send the VM into RSTATE_LOCKED
>> if it is not yet stopped on lock or is still locked on resume. That
>> would avoid redefining stop states that have no use for lock-counting
>> semantics.
>>
> 
> Which stop states would these be?  When would you want one cont to undo
> two stops?

No, cont would remain cont: the resume-after-stop command.

Locking would never be user-exposed. It would be a QEMU-internal thing,
used when there must be no resume for a certain finite while (e.g.
during migration or savevm). I think one problem with the current state
machine is that it does not differentiate between these two types of "stop".

Jan


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