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

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Date:   Mon, 6 Feb 2023 12:11:29 +0000
From:   Usama Arif <usama.arif@...edance.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Kim Phillips <kim.phillips@....com>, tglx@...utronix.de,
        rja@....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: [External] Re: [PATCH v6 07/11] x86/smpboot: Disable parallel
 boot for AMD CPUs



On 06/02/2023 08:05, David Woodhouse wrote:
> On Sun, 2023-02-05 at 22:13 +0000, Usama Arif wrote:
>>
>>
>> On 04/02/2023 22:31, David Woodhouse wrote:
>>>
>>>
>>> On 4 February 2023 18:18:55 GMT, Arjan van de Ven <arjan@...ux.intel.com> wrote:
>>>>>
>>>>> 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.
>>>>
>>>> I think that this is the right approach, at least on the initial patch series.
>>>> KISS principle; do all the easy-but-important cases first, get it stable and working
>>>> and in later series/kernels the range can be expanded.... if it matters.
>>>
>>> Agreed. I'll split it to do it only with X2APIC for the initial series, and then hold the CPUID 0x1 part back for the next phase.
>>
>> This was an interesting find! I tested the latest branch
>> parallel-6.2-rc6 and it works well. The numbers from Russ makes the
>> patch series look so much better! :)
>>
>> If we do it with x2apic only and not support non-x2apic CPUID 0x1 case,
>> maybe we apply the following diff to part 1?
> 
> Using x2apic_mode would also disable parallel boot when the CPU *does*
> have X2APIC support but the kernel just isn't using it at the moment. I
> think boot_cpu_has(X86_FEATURE_X2APIC) is the better criterion?
> 

x2apic_mode is set to 0 only in the case when nox2apic is specified in 
the kernel cmdline or if x2apic_setup fails. As 0xB leaf gets the 
"x2apic id" and not the "apic id", I thought it would be better to not 
use the x2apic id if the user doesnt want to use x2apic (cmdline), or 
the kernel fails to set it up.

Another thing I noticed from the Intel Architecture Manual CPUID—CPU 
Identification section:

"CPUID leaf 1FH is a preferred superset to leaf 0BH. Intel recommends 
first checking for the existence of Leaf 1FH before using leaf 0BH."

So I think we should switch from 0BH to using the 1FH leaf EDX register.

> I was thinking I'd tweak the 'no_parallel_bringup' command line
> argument into something that also allows us to *enable* it without
> X2APIC being supported.
> 
> But I've also been pondering the fact that this is all only for 64-bit
> anyway. It's not like we're doing it for the zoo of ancient i586 and
> even i486 machines where the APICs were hooked up with blue wire and
> duct tape.
> 
> Maybe "64-bit only" is good enough, with a command line opt-out. And
> maybe a printk pointing out the existence of that command line option
> before the bringup, just in case?
> 

I think that makes sense, the patch only has a significant impact when 
the core count is high, and x2apic was made to support higher CPU count.


>> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
>> index f53a060a899b..f6b89cf40076 100644
>> --- a/arch/x86/kernel/smpboot.c
>> +++ b/arch/x86/kernel/smpboot.c
>> @@ -1564,7 +1564,7 @@ void __init native_smp_prepare_cpus(unsigned int
>> max_cpus)
>>            * sufficient). Otherwise it's too hard. And not for SEV-ES
>>            * guests because they can't use CPUID that early.
>>            */
>> -       if (IS_ENABLED(CONFIG_X86_32) || boot_cpu_data.cpuid_level < 1 ||
>> +       if (IS_ENABLED(CONFIG_X86_32) || !x2apic_mode ||
>>               (x2apic_mode && boot_cpu_data.cpuid_level < 0xb) ||
>>               cc_platform_has(CC_ATTR_GUEST_STATE_ENCRYPT))
>>                   do_parallel_bringup = false;
>>
>>
>>
>> For reusing timer calibration, calibrate_delay ends up being used in
>> start_kernel, smp_callin, debug_calc_bogomips and check_cx686_slop. I
>> think reusing timer calibration would be ok in the first 2 uses?
>>
> 
> It already explicitly allows the calibration to be reused from another
> CPU in the same *core*, but no further. I think we'd need to be sure
> about the correctness of extending that further, and which of the mess
> of constant/invariant/we-really-mean-it-this-time TSC flags need to be
> set for that to be OK.
> 
>> but not really sure about the other 2. cx686 seems to be quite old so not sure
>> if anyone will have it to test or will ever run 6.2 kernel on it :).  I
>> guess if unsure, better to leave out initially and try and get part1 merged?
> 
> I doubt cx686 advertises constant TSC; the early ones didn't even *have* a TSC.
> Does it even support SMP?

Just checked the wiki page, and it says core count is 1 :)