On Tue, 2023-02-21 at 11:54 +0000, Usama Arif wrote: > > > On 21/02/2023 11:42, Oleksandr Natalenko wrote: > > On 21.02.2023 11:47, Usama Arif wrote: > > > On 21/02/2023 10:27, David Woodhouse wrote: > > > > > > > > On 21 February 2023 09:49:51 GMT, Oleksandr Natalenko > > > > <oleksandr@xxxxxxxxxxxxxx> wrote: > > > > > On 21.02.2023 10:06, David Woodhouse wrote: > > > > > > Why does arch/x86/kernel/acpi/sleep.c::x86_acpi_suspend_lowlevel() set > > > > > > > > > > > > initial_gs = per_cpu_offset(smp_processor_id()) ? > > > > > > > > > > > > Would it not be CPU#0 that comes back up, and should it not get > > > > > > per_cpu_offset(0) ? > > > > > > > > > > Wanna me try `initial_gs = per_cpu_offset(0);` too? > > > > > > > > > > I think it might be smp_processor_id() and not 0 incase CPU0 was > > > offline at the point the system was suspended? > > > > Is it even possible for CPU 0 to be offline, at least on x86? > > > > It is possible on x86 (using BOOTPARAM_HOTPLUG_CPU0), but I just read > the Kconfig option and it says: > > "resume from hibernate or suspend always starts from CPU0. > So hibernate and suspend are prevented if CPU0 is offline." > > so I guess switching to 0 should be ok. The interesting question is who ends up using whose stack? Leaving smpboot_control alone (for the parallel case only; we can't just leave it with the APIC ID of the last CPU started in the serial case) will make each CPU find the same CPU# and stack it used to have, from its own APIC ID and the cpuid_to_apicid[] table. I don't really grok what's happening in the non-parallel case. We leave behind the stack for the CPU that happens to be running the suspend function. And then whichever CPU comes back, it'll get *that* stack. I don't understand why the parallel bringup changes this. Does the cpuid to apicid mapping *change* on resume, if the suspending and resuming CPU are different? Do they swap stacks and CPU# somehow?
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