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" <m...@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" <m...@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
