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

[Eduardo Habkost]

On Thu, Oct 10, 2019 at 05:57:54PM +0200, Igor Mammedov wrote:
> On Thu, 10 Oct 2019 09:59:42 -0400
> "Michael S. Tsirkin" <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.
> > 
> > I'm not sure how it's even possible to create devices dynamically. Well
> > I guess it's possible with LoadTable. Is this what you had in
> > mind?
> 
> Yep. I even played this shiny toy and I can say it's very tempting one.
> On the  other side, even problem of legacy OSes not working with it aside,
> it's hard to debug and reproduce compared to static tables.
> So from maintaining pov I dislike it enough to be against it.
> 
> 
> > > 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.
> > 
> > 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?
> With current CPU hotplug interface we can handle upto UNIT32_MAX cpus, and 
> extend
> interface without need to increase IO window we are using now.
> 
> Granted IO access it not fastest compared to fwcfg in DMA mode, but we already
> doing stop machine when switching to SMM which is orders of magnitude slower.
> Consensus was to compromise on speed of CPU hotplug versus more complex and 
> more
> problematic unicast SMM mode in OVMF (can't find a particular email but we 
> have discussed
> it with Laszlo already, when I considered ways to optimize hotplug speed)

If we were designing the interface from the ground up, I would
agree with Michael.  But I don't see why we would reimplement
everything from scratch now, if just providing the
cpu_selector => cpu_hardware_id mapping to firmware is enough to
make the existing interface work.

If somebody is really unhappy with the current interface and
wants to implement a new purely fw_cfg-based one (and write the
corresponding ACPI code), they would be welcome.  I just don't
see why we should spend our time doing that now.

-- 
Eduardo