On Tue, Oct 9, 2012 at 5:04 PM, Vasilis Liaskovitis firstname.lastname@example.org wrote:
sorry for the delayed answer.
On Sat, Sep 29, 2012 at 11:13:04AM +0000, Blue Swirl wrote:
The "-dimm" option is supposed to specify the dimm/memory layout, and not create any devices.
If we don't want this new option, I have a question:
A "-device/device_add" means we create a new qdev device at startup or as a hotplug operation respectively. So, the semantics of "-device dimm,id=dimm0,size=512M,node=0,populated=on" are clear to me.
What does "-device dimm,populated=off" mean from a qdev perspective? There are 2 alternatives:
- The device is created on the dimmbus, but is not used/populated yet. Than the
activation/acpi-hotplug of the dimm may require a separate command (we used to have "dimm_add" in versions < 3). "device_add" handling always hotplugs a new qdev device, so this wouldn't fit this usecase, because the device already exists. In this case, the actual "acpi hotplug" operation is decoupled from qdev device creation.
The bus exists but the devices do not, device_add would add DIMMs to the bus. This matches PCI bus created by the host bridge and PCI device hotplug.
A more complex setup would be dimm bus, dimm slot devices and DIMM devices. The intermediate slot device would contain one DIMM device if plugged.
interesting, I haven't thought about this alternative. It does sounds overly complex, but a dimmslot / dimmdevice splitup could consolidate hotplug semantic differences between populated=on/off. Something similar to the dimmslot device is already present in v3 (dimmcfg structure), but it's not a qdev visible device. I 'd rather avoid the complication, but i might revisit this idea.
The memory controller could be able to also enable and disable slots independently to their population state.
- The dimmdevice is not created when "-device dimm,populated=off" (this would
require some ugly checking in normal -device argument handling). Only the dimm layout is saved. The hotplug is triggered from a normal device_add later. So in this case, the "acpi hotplug" happens at the same time as the qdev hotplug.
Do you see a simpler alternative without introducing a new option?
Using the "-dimm" option follows the second semantic and avoids changing the "-device" semantics. Dimm layout description is decoupled from dimmdevice creation, and qdev hotplug coincides with acpi hotplug.
Maybe even the dimmbus device shouldn't exist by itself after all, or it should be pretty much invisible to users. On real HW, the memory controller or south bridge handles the memory. For i440fx, it's part of the same chipset. So I think we should just add qdev properties to i440fx to specify the sizes, nodes etc. Then i440fx should create the dimmbus device unconditionally using the properties. The default properties should create a sane configuration, otherwise -global i440fx.dimm_size=512M etc. could be used. Then the bus would be populated as before or with device_add.
hmm the problem with using only i440fx properties, is that size/nodes look dimm specific to me, not chipset-memcontroller specific. Unless we only allow uniform size dimms. Is it possible to have a dynamic list of sizes/nodes pairs as properties of a qdev device?
I don't think so, but probably there's a limit of DIMMs that real controllers have, something like 8 max.
Also if there is no dimmbus, and instead we have only links<> from i440fx to dimm-devices, would the current qdev hotplug API be enough?
I'd just disable hotplug if there is no dimmbus (ISA PC?).
I am currently leaning towards this: i440fx unconditionally creates the dimmbus. Users don't have to specify the bus (i assume this is what you mean by "dimmbus should be invisible to the users")
We only use "-device dimm" to describe dimms. With "-device dimm,populated=off", only the dimm config layout will be saved in the dimmbus. The hotplug is triggered from a normal device_add later (same as pci hotplug).