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

[Eduardo Habkost <ehabkost@redhat.com>]

On Thu, Oct 10, 2019 at 03:39:12PM +0200, Igor Mammedov wrote:
> On Thu, 10 Oct 2019 05:56:55 -0400
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Wed, Oct 09, 2019 at 09:22:49AM -0400, Igor Mammedov wrote:
> > > As an alternative to passing to firmware topology info via new fwcfg files
> > > so it could recreate APIC IDs based on it and order CPUs are enumerated,
> > > 
> > > extend CPU hotplug interface to return APIC ID as response to the new command
> > > CPHP_GET_CPU_ID_CMD.  
> > 
> > One big piece missing here is motivation:
> I thought the only willing reader was Laszlo (who is aware of context)
> so I skipped on details and confused others :/
> 
> > Who's going to use this interface?
> In current state it's for firmware, since ACPI tables can cheat
> by having APIC IDs statically built in.
> 
> If we were creating CPU objects in ACPI dynamically
> we would be using this command as well. It would save
> us quite a bit space in ACPI blob but it would be a pain
> to debug and diagnose problems in ACPI tables, so I'd rather
> stay with static CPU descriptions in ACPI tables for the sake
> of maintenance.
> 
> > So far CPU hotplug was used by the ACPI, so we didn't
> > really commit to a fixed interface too strongly.
> > 
> > Is this a replacement to Laszlo's fw cfg interface?
> > If yes is the idea that OVMF going to depend on CPU hotplug directly then?
> > It does not depend on it now, does it?
> It doesn't, but then it doesn't support cpu hotplug,
> OVMF(SMM) needs to cooperate with QEMU "and" ACPI tables to perform
> the task and using the same interface/code path between all involved
> parties makes the task easier with the least amount of duplicated
> interfaces and more robust.
> 
> Re-implementing alternative interface for firmware (fwcfg or what not)
> would work as well, but it's only question of time when ACPI and
> this new interface disagree on how world works and process falls
> apart.
> 
> > If answers to all of the above is yes, then I don't really like it: it
> > is better to keep all paravirt stuff in one place, namely in fw cfg.
> Lets discuss, what cpu hotplug fwcfg interface could look like in 
>  [PATCH 3/4] hw/i386: add facility to expose CPU topology over  fw-cfg
> mail thread and clarify (dis)likes with concrete reasons.
> 
> So far I managed to convince myself that we ought to reuse
> and extend current CPU hotplug interface with firmware features,
> to endup with consolidated cpu hotplug process without
> introducing duplicate ABIs, but I could be wrong so
> lets see if fwcfg will be the better approach.
> 

I was more inclined towards the approach in this patch, because I
see it as just a bug fix in the CPU hotplug interface (which
should have been using the hardware CPU identifier as the CPU
selector since the beginning).

Providing the missing information in fw_cfg isn't necessarily
bad, but please document it explicitly as a
  hotplug_cpu_selector => cpu_hardware_id
mapping, so people won't use "CPU index" as a generic identifier
elsewhere.

-- 
Eduardo