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 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
