linux-kernel - Re: [External] Re: [PATCH v9 0/8] Parallel CPU bringup for x86

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Date:   Wed, 22 Feb 2023 08:19:27 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Usama Arif <usama.arif@...edance.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Oleksandr Natalenko <oleksandr@...alenko.name>
Cc:     Kim Phillips <kim.phillips@....com>, arjan@...ux.intel.com,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, x86@...nel.org, pbonzini@...hat.com,
        paulmck@...nel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, rcu@...r.kernel.org, mimoja@...oja.de,
        hewenliang4@...wei.com, thomas.lendacky@....com, seanjc@...gle.com,
        pmenzel@...gen.mpg.de, fam.zheng@...edance.com,
        punit.agrawal@...edance.com, simon.evans@...edance.com,
        liangma@...ngbit.com,
        "Limonciello, Mario" <Mario.Limonciello@....com>,
        Piotr Gorski <piotrgorski@...hyos.org>
Subject: Re: [External] Re: [PATCH v9 0/8] Parallel CPU bringup for x86_64

On Wed, 2023-02-22 at 00:00 +0000, Usama Arif wrote:
> 
> > diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
> > index 33c0d5fd8af6..72b9375fec7c 100644
> > --- a/arch/x86/include/asm/smp.h
> > +++ b/arch/x86/include/asm/smp.h
> > @@ -208,4 +208,6 @@ extern unsigned int smpboot_control;
> >    #define STARTUP_APICID_CPUID_0B      0x40000000
> >    #define STARTUP_APICID_CPUID_01      0x20000000
> >    
> > +#define STARTUP_PARALLEL_MASK          0x60000000
> > +
> 
> Probably could define STARTUP_PARALLEL_MASK as STARTUP_APICID_CPUID_0B | 
> STARTUP_APICID_CPUID_01 instead? otherwise if its a separate bit, it 
> needs to be set in native_smp_prepare_cpus as well for this to work?

It is CPUID_0B | CPUID_01, unless I was being really stupid last night. 
Sips coffee... yep, 6 = 4 | 2. I could have made that more obvious with
a bit more typing, I suppose.

However, I don't think this approach is correct. The idle thread stacks
for the other CPUs are *unused* because those CPUs are offline. So we
can use them with impunity, and when those CPUs come back online
they'll call into cpu_startup_entry(CPUHP_AP_ONLINE_IDLE) using the
correct stack.

But the BSP/CPU0 is different. It hasn't actually been taken offline,
and its idle thread context is still in cpu_startup_entry(CPUHP_ONLINE)
which got called from rest_init().

In testing I probably got away with it because we're only using the
*top* of the stack, don't use anything of the red zone, and thus don't
actually bother the true idle thread which is never going to return.
But I don't think it's correct; we really ought to have that temp_stack
unless we're going to refactor the wakeup_64 code to *become* the idle
thread just as startup_secondary() does, and *schedule* to the context
that was saved in the suspend code.

That might be an interesting cleanup, and let us use the normal
__switch_to() to save and restore a bunch of context which is currently
done by hand.

But not today.

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