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+++ This bug was initially created as a clone of Bug #1390346 +++

QEMU reserves only 32-bit memory range for hotplug. The range can be rather limited and not enough to hot-plug devices with large BARs.

Add a parameter to QEMU to reserve a 64-bit range in CRS
starting after the memory ranged reserved for memory hotplug.

Check that all the range is addressable by VM's CPU.

Add libvirt support for the new command line parameters.

Hi,

In order to be able to hot-plug PCI devices, QEMU needs to reserve some address space to be mapped to PCI devices registers.

QEMU today reserves space only on 32-bit area (< 4G) memory. The problem is the "window" reserved is not always enough, especially if the VM has a lot of devices (they also use the same pool).
However (>4G) memory space is huge and the most part is not actually used, the only limit is the CPU addressable bits.

The question is how much space to reserve? Reserving too much can limit the migration only to the host where the VMs can support the same addressable bits.

Who can "guess" how much reservation we "need"? I suppose libvirt/nova, they can query the pyhsical PCI devices that may be attached in the future, the hosts CPU limitations and so on.

However, this is too low level and we want to come up with a solution that does not involve libvirt/nova, at least we will try.

Thanks,
Marcel

(In reply to Marcel Apfelbaum from comment #3)
> 
> However, this is too low level and we want to come up with a solution that
> does not involve libvirt/nova, at least we will try.

Ok, please reopen if there is anything for libvirt.

Thanks,
J.