Re: [Qemu-devel] [PATCH 00/16 RFC v6] ACPI memory hotplug

[Hu Tao <hutao@cn.fujitsu.com>]

On Tue, Jul 23, 2013 at 06:22:56PM +0200, Igor Mammedov wrote:
> As opposed to previous approach,
> This series allows to hotplug 'arbitrary' DIMM devices specifying size,
> NUMA node mapping, slot and address where to map it, at runtime.
> 
> Due to ACPI limitation there is need to specify a number of possible
> DIMM devices. For this task -m option was extended to support
> following format:
> 
>   -m [mem=]RamSize[,slots=N,maxmem=M]
> 
> To allow memory hotplug user must specify a pair additional parameters:
>     'slots' - number of possible increments
>     'maxmem' - max possible total memory size QEMU is allowed to use,
>                including RamSize.
> 
> minimal monitor command syntax to hotplug DIMM device:
> 
>   device_add dimm,id=dimmX
> 
> DIMM device provides following properties that could be used with
> device_add / -device to alter default behavior:
> 
>   id    - unique string identifying device [mandatory]
>   slot  - number in range [0-slots) [optional], if not specified
>           the first free slot is used
>   node  - NUMA node id [optional] (default: 0)
>   size  - amount of memory to add [optional] (default: 1Gb)
>   start - guest's physical address where to plug DIMM [optional],
>           if not specified the first gap in hotplug memory region
>           that fits DIMM is used
> 
>  -device option could be used for adding potentially hotunplugable DIMMs
> and also for specifying hotplugged DIMMs in migration case (not tested).
> 
> Current implementation supports only x86-64 variant and places hotplug
> memory region above 4Gb before 64-bit PCI hole.
> 
> Tested guests:
>  - Fedora 19x64
>  - Windows 2012DCx64
>  - Windows 2008DCx64
> 
> Known limitations/bugs/TODOs:
>  - only hot-add supported
>  - q35 is not supported yet
>  - max number of supported DIMM devices 255 (due to ACPI object name
>    limit), could be increased creating several containers and putting
>    DIMMs there. (exercise for future) 
>  - failed hotplug action consumes 1 slot (device_add doesn't delete
>    device if realize failed)
>  - e820 table doesn't include DIMM devices added with -device /
>    (or after reboot devices added with device_add)
>  - Windows 2008 remembers DIMM configuration, so if DIMM with other
>    start/size is added into the same slot, it refuses to use it insisting
>    on old mapping.

With this series we can hotplug memory of arbitrary size, but Linux
expects a minimum size of hotpluggable memory. Take 128M in x86_64 for
example, if first hotplug 64M memory (less than 128M) than we can't add
another 64M memory. So the question is should we have a lower limit of
hotplugged memory in qemu (which is easy but it's not qemu's problem,
and different OS/hardware may have different limits)? Or maybe we can
fix it in Linux (which is difficult and I'm not sure it'll casue any
compatibility problem)?