linux-kernel - Re: [PATCH v10 6/8] x86/smpboot: Support parallel startup of secondary CPUs

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Date:   Fri, 24 Feb 2023 15:46:27 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Yuan Yao <yuan.yao@...ux.intel.com>,
        Usama Arif <usama.arif@...edance.com>
Cc:     tglx@...utronix.de, kim.phillips@....com, piotrgorski@...hyos.org,
        oleksandr@...alenko.name, 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
Subject: Re: [PATCH v10 6/8] x86/smpboot: Support parallel startup of
 secondary CPUs

On Fri, 2023-02-24 at 13:59 +0800, Yuan Yao wrote:
> 
> > +        * Make sure only one CPU fiddles with the realmode stack
> > +        */
> > +.Llock_rm:
> > +       btl     $0, tr_lock
> > +       jnc     2f
> > +       pause
> > +       jmp     .Llock_rm
> > +2:
> > +       lock
> > +       btsl    $0, tr_lock
> > +       jc      .Llock_rm
> > +
> 
> Looks these changes should be applied to trampoline_start64()
> yet, which is used for boot up APs when apic->wakeup_secondary_cpu_64
> is available, e.g when ACPI_MADT_TYPE_MULTIPROC_WAKEUP is available.
> 
> One case I know is the INTEL TD guest, which using the MADT wakeup
> for AP wake up now.

Yeah.

I think we could probably pull that all out into a separate asm
"load_realmode_esp" function which takes the lock and actually does the
'movl $rm_stack_end, %esp'.

Then we call it from all the places which currently use $rm_stack_end,
including sev_es_trampoline_start (even though we currently disable
parallel startup there because CPUID doesn't work that early).

Oh... except of course it can't be a function because we haven't got a
stack, have we? It's a macro.

Usama, are you happy using .macro in gas? 

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