Message-ID: <d081ffa14a8c1f9f68e393a56520da764721a110.camel@infradead.org>
Subject: Re: [External] Re: [PATCH v9 0/8] Parallel CPU bringup for x86_64
From:   David Woodhouse <dwmw2@infradead.org>
To:     Usama Arif <usama.arif@bytedance.com>,
        Oleksandr Natalenko <oleksandr@natalenko.name>
Cc:     Kim Phillips <kim.phillips@amd.com>, tglx@linutronix.de,
        arjan@linux.intel.com, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com, hpa@zytor.com, x86@kernel.org,
        pbonzini@redhat.com, paulmck@kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        rcu@vger.kernel.org, mimoja@mimoja.de, hewenliang4@huawei.com,
        thomas.lendacky@amd.com, seanjc@google.com, pmenzel@molgen.mpg.de,
        fam.zheng@bytedance.com, punit.agrawal@bytedance.com,
        simon.evans@bytedance.com, liangma@liangbit.com,
        "Limonciello, Mario" <Mario.Limonciello@amd.com>,
        Piotr Gorski <piotrgorski@cachyos.org>
Date:   Tue, 21 Feb 2023 13:22:20 +0000
In-Reply-To: <049adfae-f4a0-349e-323a-52299969ef99@bytedance.com>
References: <20230215145425.420125-1-usama.arif@bytedance.com>
         <2a67f6cf18dd2c1879fad9fd8a28242918d3e5d2.camel@infradead.org>
         <982e1d6140705414e8fd60b990bd259a@natalenko.name>
         <715CBABF-4017-4784-8F30-5386F1524830@infradead.org>
         <67dbc69f-b712-8971-f1c9-5d07f506a19c@amd.com>
         <42dc683e2846ae8fc1e09715aaf7884660e1a386.camel@infradead.org>
         <37c18c3aeea2e558633b6da6886111d0@natalenko.name>
         <5A3B7074-0C6D-472B-803B-D76541828C1F@infradead.org>
         <3d8ed6e157df10c5175c636de0e21849@natalenko.name>
         <5c557f9b6f55dc2a612ee89142971298e6ae12d8.camel@infradead.org>
         <ee0d0d971a3095d6a1e96ad4f1ba32d2@natalenko.name>
         <5b8f9c89f7015fa80c966c6c7f6fa259db6744f8.camel@infradead.org>
         <ce731b5a4a53680b4840467977b33d9a@natalenko.name>
         <85ceb3f92abf3c013924de2f025517372bed19c0.camel@infradead.org>
         <3e5944de08ef0d23584d19bad7bae66c@natalenko.name>
         <26E5DC9C-0F19-4E4F-9076-04506A197374@infradead.org>
         <d91f96eb-86a1-c794-761d-a96e39076b13@bytedance.com>
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         <049adfae-f4a0-349e-323a-52299969ef99@bytedance.com>
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On Tue, 2023-02-21 at 11:54 +0000, Usama Arif wrote:
>=20
>=20
> 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:
> > > >=20
> > > > On 21 February 2023 09:49:51 GMT, Oleksandr Natalenko=20
> > > > <oleksandr@natalenko.name> wrote:
> > > > > On 21.02.2023 10:06, David Woodhouse wrote:
> > > > > > Why does arch/x86/kernel/acpi/sleep.c::x86_acpi_suspend_lowleve=
l() set
> > > > > >=20
> > > > > > =C2=A0=C2=A0=C2=A0=C2=A0 initial_gs =3D per_cpu_offset(smp_proc=
essor_id()) ?
> > > > > >=20
> > > > > > Would it not be CPU#0 that comes back up, and should it not get
> > > > > > per_cpu_offset(0) ?
> > > > >=20
> > > > > Wanna me try `initial_gs =3D per_cpu_offset(0);` too?
> > > >=20
> > >=20
> > > I think it might be smp_processor_id() and not 0 incase CPU0 was=20
> > > offline at the point the system was suspended?
> >=20
> > Is it even possible for CPU 0 to be offline, at least on x86?
> >=20
>=20
> It is possible on x86 (using BOOTPARAM_HOTPLUG_CPU0), but I just read
> the Kconfig option and it says:
>=20
> "resume from hibernate or suspend always starts from CPU0.
> So hibernate and suspend are prevented if CPU0 is offline."
>=20
> 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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