linux-kernel - Re: [PATCH v13 00/11] Parallel CPU bringup for x86

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Date:   Tue, 07 Mar 2023 22:27:45 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Tom Lendacky <thomas.lendacky@....com>,
        Usama Arif <usama.arif@...edance.com>, tglx@...utronix.de,
        kim.phillips@....com, brgerst@...il.com,
        "Rapan, Sabin" <sabrapan@...zon.com>
Cc:     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,
        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 v13 00/11] Parallel CPU bringup for x86_64

On Tue, 2023-03-07 at 16:22 -0600, Tom Lendacky wrote:
> 
> I did some Qemu/KVM testing. One thing I noticed is that on AMD, CPUID 0xB 
> EAX will be non-zero only if SMT is enabled. So just booting some guests 
> without CPU topology never did parallel booting ("smpboot: Disabling 
> parallel bringup because CPUID 0xb looks untrustworthy"). I would imagine 
> a bare-metal system that has diabled SMT will not do parallel booting, too 
> (but I haven't had time to test that).

Interesting, thanks. Should I change to checking for *both* EAX and EBX
being zero? That's what I did first, after reading only the Intel SDM.
But I changed to only EAX because the AMD doc only says that EAX will
be zero for unsupported leaves.

> I did have to change "vmgexit" to a "rep; vmmcall" based on my binutils 
> and the IS_ENABLED(CONFIG_AMD_MEM_ENCRYPT) change I mentioned before. But 
> with that, I was able to successfully boot 64 vCPU SEV-ES and SEV-SNP
> guests using parallel booting.

Thanks. I'll look at retconning that rework of can_parallel_bringup()
out into a separate function, into the earlier parts of the series.

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