Re: [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface

[Michael S. Tsirkin]

On Thu, Oct 10, 2019 at 05:57:54PM +0200, Igor Mammedov wrote:
> > Then we should consider switching acpi to use fw cfg.
> > Or build another interface that can scale.
> 
> Could be an option, it would be a pain to write a driver in AML for fwcfg 
> access though
> (I've looked at possibility to access fwcfg from AML about a year ago and 
> gave up.
> I'm definitely not volunteering for the second attempt and can't even give an 
> estimate
> it it's viable approach).
> 
> But what scaling issue you are talking about, exactly?

Just this: each new thing we add is an ad-hoc data structure with
manually maintained backwards compatibility and no built-in discovery.

fw cfg has built-in discovery and we've finally managed to
handle compatibility reasonably well.

PV is already very problematic.  Spreading PV code all over the place
like this is a very bad idea.  For you CPU hotplug is something that you
keep in mind first of all, but someone bringing up a new platform
already has a steep hill to climb.  Adding tons of custom firmware is
not helping things.

-- 
MST