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

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Oct 11, 2019 at 10:01:42AM +0200, Laszlo Ersek wrote:
> On 10/10/19 21:20, Eduardo Habkost wrote:
> > 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" <mst@redhat.com> 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" <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.
> >>>
> >>> 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.
> 
> Let me re-iterate the difficulties quickly:
> 
> - DMA-based fw_cfg is troublesome in SEV guests (do you want to mess
> with page table entries in AML methods? or pre-allocate an always
> decrypted opregion? how large?)
> 
> - IO port based fw_cfg does not support writes (and I reckon that, when
> the *OS* handles a hotplug event, it does have to talk back to QEMU)
> 
> - the CPU hotplug AML would have to arbitrate with Linux's own fw_cfg
> driver (which exposes fw_cfg files to userspace, yay! /s)
> 
> In the phys world, CPU hotplug takes dedicated RAS hardware. Shoehorning
> CPU hotplug into *firmware* config, when in two use cases [*], the
> firmware shouldn't even know about CPU hotplug, feels messy.
> 
> [*] being (a) SeaBIOS, and (b) OVMF built without SMM

I agree. So ACPI should use a dedicated interface.

> > I just don't see why we should spend our time doing that now.
> 
> I have to agree, we're already spread thin.
> 
> ... I must admit: I didn't expect this, but now I've grown to *prefer*
> the CPU hotplug register block!
> 
> Laszlo

OK, send an ack then.

-- 
MST