linux-kernel - Re: [PATCH v6 07/11] x86/smpboot: Disable parallel boot for AMD CPUs

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Date:   Sat, 04 Feb 2023 15:40:32 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Kim Phillips <kim.phillips@....com>,
        Usama Arif <usama.arif@...edance.com>, tglx@...utronix.de,
        arjan@...ux.intel.com
Cc:     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, Mario Limonciello <Mario.Limonciello@....com>
Subject: Re: [PATCH v6 07/11] x86/smpboot: Disable parallel boot for AMD CPUs

On Fri, 2023-02-03 at 13:48 -0600, Kim Phillips wrote:
> 
> I'd like to nack this, but can't (and not because it doesn't have
> commit text)
> 

So, I *think* that worked precisely as designed; thanks for letting it
grab your attention :)

> If I:
> 
>   - take dwmw2's parallel-6.2-rc6 branch (commit 459d1c46dbd1)
>   - remove the set_cpu_bug(c, X86_BUG_NO_PARALLEL_BRINGUP) line from amd.c
> 
> Then:
> 
>   - a Ryzen 3000 (Picasso A1/Zen+) notebook I have access to fails to boot.
>   - Zen 2,3,4-based servers boot fine
>   - a Zen1-based server doesn't boot.

I've changed it to use CPUID 0xb only if we're actually in x2apic mode,
which Boris tells me won't be the case on Zen1 because that doesn't
support X2APIC.

When we're not in x2apic mode, we can use CPUID 0x1 because the 8 bits
of APIC ID we find there are perfectly sufficient.

New tree in the same place as before, commit ce7e2d1e046a for the
parallel-6.2-rc6-part1 tag and 17bbd12ee03 for parallel-6.2-rc6.

However...

Even though we *can* support non-X2APIC processors, we *might* want to
play it safe and not go back that far; only enabling parallel bringup
on machines with X2APIC which roughly correlates with "lots of CPUs"
since that's where the benefit is.

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