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

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Message-ID: <39f23da7-1e77-4535-21a6-00f77a382ae5@amd.com>
Date:   Tue, 7 Mar 2023 16:55:31 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     David Woodhouse <dwmw2@...radead.org>,
        Usama Arif <usama.arif@...edance.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "Phillips, Kim" <kim.phillips@....com>,
        "brgerst@...il.com" <brgerst@...il.com>,
        "Rapan, Sabin" <sabrapan@...zon.com>
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        "oleksandr@...alenko.name" <oleksandr@...alenko.name>,
        "arjan@...ux.intel.com" <arjan@...ux.intel.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
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Subject: Re: [PATCH v13 00/11] Parallel CPU bringup for x86_64

On 3/7/23 16:27, David Woodhouse wrote:
> 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.

 From a baremetal perspective, I think that works. Rome was the first 
generation to support x2apic, and the PPR for Rome states that 0's are 
returned in all 4 registers for undefined function numbers.

For virtualization, at least Qemu/KVM, that also looks to be a safe test.

Thanks,
Tom

> 
>> 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.