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

[Thomas Gleixner]

David!

On Tue, Feb 07 2023 at 10:04, David Woodhouse wrote:
> On Tue, 2023-02-07 at 01:23 +0100, Thomas Gleixner wrote:
>> > 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.
>> 
>> Is that worth the trouble?
>
> Well, that's what was being debated. I think the conclusion that was
> bring reached was that it *is* worth the trouble, because there will be
> a number of physical and especially virtual machines which have a high
> CPU count but which don't actually use X2APIC mode. And which might not
> even *support* CPUID 0xb.
>
> So using CPUID 0x1 when there is no APIC ID greater than 254 does seem
> to make sense.

Fair enough.

>> > 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.
>> 
>> The parallel bringup code is complex enough already, so please don't
>> optimize for the non-interesting case in the first place. When this has
>> stabilized then the CPUID 0x1 mechanism can be added if anyone thinks
>> it's interesting. KISS is still the best engineering principle.
>
> Actually it ends up being trivial. It probably makes sense to keep it
> in there even if it can only be exercised by a deliberate opt-in on
> older CPUs. I reworked the register usage from your original anyway,
> which helps a little.
>
> 	testl	$STARTUP_APICID_CPUID_0B, %edx
> 	jnz	.Luse_cpuid_0b
> 	testl	$STARTUP_APICID_CPUID_01, %edx
> 	jnz	.Luse_cpuid_01
> 	andl	$0x0FFFFFFF, %edx
> 	jmp	.Lsetup_AP
>
> .Luse_cpuid_01:
> 	mov	$0x01, %eax
> 	cpuid
> 	mov	%ebx, %edx
> 	shr	$24, %edx
> 	jmp	.Lsetup_AP
>
> .Luse_cpuid_0b:
> 	mov	$0x0B, %eax
> 	xorl	%ecx, %ecx
> 	cpuid
>
> .Lsetup_AP:
> 	/* EDX contains the APICID of the current CPU */

That looks trivial enough. So no objections from my side. Not sure
whether this needs a special opt-in though. We probably want an opt-out
for the parallel bringup mode for diagnosis purposes anyway and that
should be good enough for a start.

Thanks,

        tglx