On Thu, 10 Oct 2019 11:16:52 -0300 Eduardo Habkost <ehabk...@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. 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. > > > > > If answers to all of the above is yes, then I don't really like it: it > > > is better to keep all paravirt stuff in one place, namely in fw cfg. > > Lets discuss, what cpu hotplug fwcfg interface could look like in > > [PATCH 3/4] hw/i386: add facility to expose CPU topology over fw-cfg > > mail thread and clarify (dis)likes with concrete reasons. > > > > So far I managed to convince myself that we ought to reuse > > and extend current CPU hotplug interface with firmware features, > > to endup with consolidated cpu hotplug process without > > introducing duplicate ABIs, but I could be wrong so > > lets see if fwcfg will be the better approach. > > > > I was more inclined towards the approach in this patch, because I > see it as just a bug fix in the CPU hotplug interface (which > should have been using the hardware CPU identifier as the CPU > selector since the beginning). > > Providing the missing information in fw_cfg isn't necessarily > bad, but please document it explicitly as a > hotplug_cpu_selector => cpu_hardware_id > mapping, so people won't use "CPU index" as a generic identifier > elsewhere. Currently cpu_selector is UID (or whatever you'd like to name it) for a CPU instance in ACPI tables. It just happens to be non sparse range [0..max_cpus) and was just a convenient way to make up IDs and handle them on hw side (requires simple array). Sure I'll document it as such to avoid mis-understanding, plus a bunch of other fixes to the spec.
