Re: [PATCH 3/4] hw/i386: add facility to expose CPU topology over fw-cfg

[Laszlo Ersek <lersek@redhat.com>]

On 10/10/19 12:01, Michael S. Tsirkin wrote:
> On Tue, Oct 08, 2019 at 05:59:31PM +0200, Igor Mammedov wrote:

>> So far there were no need for it as all possible cpus are
>> described in ACPI tables passed to guest, but I'm not going
>> to suggest to parse them on firmware side as it's too complicated :)
> 
> We can always add a QEMU specific data table by the way.
> Format would be up to us and would be easy to parse.
> I don't see a big advantage as compared to fw cfg though.

I'd like to comment just on this part.

*If* we decide to expose the information through some kind of data table
(as opposed to the modern CPU hotplug register block), then the
representation *must* be a dedicated fw_cfg blob. It cannot be an ACPI
table.

The reason is that *selecting* the fw_cfg blob that contains the ACPI
linker/loader script is a very specific action (it re-generates the ACPI
payload, with dependencies on assigned PCI resources). Therefore, it is
done in a super-particular spot in the firmware.

On the other hand, the "possible CPUs count" is needed much earlier than
that. I need to fetch that info way before PCI resource assignment
appears on the radar.

So please let us stick with "ACPI is only for the guest OS to read" rule
-- it's not only a parsing convenience for the firmware, but a real
necessity.

Thanks
Laszlo