[Qemu-devel] Semantics of DeviceState::realized and BlockDriverState

[Anthony Liguori]

Hi,

I wanted to start a central thread as this discussion is spread in two places 
right now.

I think I'm becoming convinced that realized belongs in DeviceState and that 
BlockDriverState does not have a realized equivalent.

To me, realized represents Vcc.  When realized=true, the guest has power and is 
active.  When realized=false, the guest has lost power.  The realize() event is 
the rising edge of Vcc, unrealized() is the falling edge.

realize() should be used to take any actions that require all parameters to be 
set that need to happen before the guest has power.  This later clause is 
extremely important.  unrealize() should be used to unset anything setup in 
realize().

The destructor being invoked does not imply that unrealize() has happened.

I think BlockDriverState wants two separate signals: an opened signal and an 
attached signal.

With BDS, there are a few phases:

opened = false
attached = false

All properties can be set here and no side effects occur.

It is not valid to do a read/write request to a BDS in this state.

opened = true
attached = false

Properties that affect file opening (flags and filename) cannot be changed after 
this point.  Depending on the contents of the file, a backing_file property may 
be created after opened = true.  The backing_file that gets created does *not* 
automatically have opened = true.  A user explicitly needs to set that.

This behavior is necessary to allow overriding backing files.

It is not valid to do a read/write request to a BDS in this state.

opened = true
attached = true

This means that the device is in-use by something in QEMU.  Requests may be in 
flight as read/write requests are now allowed to be issued.

Most properties cannot be changed in this state (presumably).

opened = false
attached = true

Invalid state.

So there are three valid states and none of them map directly to the concept of 
realized for devices.

I think this argues pretty clearly for realize() to not live in Object and 
instead to allow base classes to implement whatever properties make sense to them.

Regard,

Anthony Liguori