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

[Igor Mammedov]

On Thu, 10 Oct 2019 11:16:52 -0300
Eduardo Habkost <address@hidden> wrote:

> 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" <address@hidden> 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.

Currently cpu_selector is UID (or whatever you'd like to name it)
for a CPU instance in ACPI tables. It just happens to be non sparse
range [0..max_cpus) and was just a convenient way to make up IDs
and handle them on hw side (requires simple array).

Sure I'll document it as such to avoid mis-understanding,
plus a bunch of other fixes to the spec.