On Thu, 10 Oct 2019 16:20:39 -0300 Eduardo Habkost <ehabk...@redhat.com> 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. I just don't > see why we should spend our time doing that now. Right, we can give fwcfg a shot next time we try to allocate new register block for a new PV interface, assuming it suits interface requirements.
