Re: [Qemu-devel] [Linaro-acpi] [RFC PATCH 0/7] hw/arm/virt: Dynamic ACPI v5.1 table generation

[Paolo Bonzini <pbonzini@redhat.com>]

On 06/11/2014 07:53, Hanjun Guo wrote:
>> So the important question is _why_ the guest needs to see an ACPI
>> environment. What exactly can ACPI provide to the guest that DT does not
>> already provide, and why is that necessary? What infrastrucutre is
>> needed for that use case?
> 
> There is important feature called system device dynamic reconfiguration,
> you know, hot-add/remove, if a gust need more/less memory or CPU, can we
> add or remove them dynamically with DT? ACPI can do this, but I have no
> idea if DT can. (Sorry, I don't know much about DT)

Indeed hot-add/remove is the single biggest AML user in x86 QEMU.
Whether you really need it, it depends on what you are adding/removing.

For PCI there is no problem.  We can use PCIe from the beginning, and
use PCIe hotplug support that is already in QEMU.

Memory and CPU are more problematic.  For memory we could perhaps use a
PCI memory device, though I'm not sure if that would require drivers in
the OS or everything just works.

CPU hotplug, however, probably requires AML.  Of course it can be
generated in the firmware, like we used to do for x86, but Igor
explained why it wasn't a great idea.  That said, one of the problems
("never ending expansion of PV QEMU-BIOS interface") could be less
important since ARM DT is a better interface than x86 fw_cfg.

Paolo