On 10/09/19 13:13, Igor Mammedov wrote: > On Tue, 8 Oct 2019 20:58:30 +0200 > Laszlo Ersek <ler...@redhat.com> wrote: > >> Eduardo, Igor, >> >> On 10/08/19 12:52, Laszlo Ersek wrote: >>> FW_CFG_MAX_CPUS exposes the (exclusive) maximum APIC ID to guest firmware, >>> due to historical reasons. That value is not useful to edk2, however. For >>> supporting VCPU hotplug, edk2 needs: >>> >>> - the boot CPU count (already exposed in FW_CFG_NB_CPUS), >>> >>> - and the maximum foreseen CPU count (tracked in >>> "MachineState.smp.max_cpus", but not currently exposed). >>> >>> Add a new fw-cfg file to expose "max_cpus". >>> >>> While at it, expose the rest of the topology too (die / core / thread >>> counts), because I expect that the VCPU hotplug feature for OVMF will >>> ultimately need those too, and the data size is not large. >> >> In fact, it seems like OVMF will have to synthesize the new >> (hot-plugged) VCPU's *APIC-ID* from the following information sources: >> >> - the topology information described above (die / core / thread counts), and >> >> - the "modern" CPU hotplug interface (docs/specs/acpi_cpu_hotplug.txt). > > In general duplicating cpu_index+topo => apic id in firmware I > consider as a really bad idea (even ignoring cpu_index > which I were trying to get rid of in QEMU), it's going to break > when algorithms diverge and it will be never ending race.
OK. > Topology is rather messy business, not only arch specific but also > cpu specific (ex: on my review TODO list, there is a series for > fixing broken EPYCs topo). Who knows what other variables would be > add dependencies for calculating APIC IDs down the road. > > I also consider to re-use CPU hotplug interface on ARM, which will > bring its own set of algorithms. > > Let's instead add a command to CPU hotplug interface to return > APIC ID (which QEMU already calculated) and later MPIDR (ARM) > for selected CPU, so firmware could get it while enumeration CPUs > via that interface. Sounds good to me, thanks. I'll stay tuned for your patches! :) Thanks! Laszlo
