Message-ID: <1975308c952236895f2d8f0e56af9db288eaf330.camel@infradead.org>
Subject: Re: [PATCH v13 00/11] Parallel CPU bringup for x86_64
From:   David Woodhouse <dwmw2@infradead.org>
To:     Tom Lendacky <thomas.lendacky@amd.com>,
        Usama Arif <usama.arif@bytedance.com>, tglx@linutronix.de,
        kim.phillips@amd.com, brgerst@gmail.com,
        "Rapan, Sabin" <sabrapan@amazon.com>
Cc:     piotrgorski@cachyos.org, oleksandr@natalenko.name,
        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,
        seanjc@google.com, pmenzel@molgen.mpg.de, fam.zheng@bytedance.com,
        punit.agrawal@bytedance.com, simon.evans@bytedance.com,
        liangma@liangbit.com
Date:   Tue, 07 Mar 2023 22:27:45 +0000
In-Reply-To: <3bfbbd92-b2ed-8189-7b57-0533f6c87ae7@amd.com>
References: <20230302111227.2102545-1-usama.arif@bytedance.com>
         <faa0eb3bb8ba0326d501516a057ab46eaf1f3c05.camel@infradead.org>
         <effbb6e2-c5a1-af7f-830d-8d7088f57477@amd.com>
         <269ed38b5eed9c3a259c183d59d4f1eb5128f132.camel@infradead.org>
         <0c56683a-c258-46f6-056e-e85da8a557db@amd.com>
         <3bfbbd92-b2ed-8189-7b57-0533f6c87ae7@amd.com>
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On Tue, 2023-03-07 at 16:22 -0600, Tom Lendacky wrote:
>=20
> I did some Qemu/KVM testing. One thing I noticed is that on AMD, CPUID 0x=
B=20
> EAX will be non-zero only if SMT is enabled. So just booting some guests=
=20
> without CPU topology never did parallel booting ("smpboot: Disabling=20
> parallel bringup because CPUID 0xb looks untrustworthy"). I would imagine=
=20
> a bare-metal system that has diabled SMT will not do parallel booting, to=
o=20
> (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=
=20
> and the IS_ENABLED(CONFIG_AMD_MEM_ENCRYPT) change I mentioned before. But=
=20
> 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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