[MODERATED] Re: [patch V4 13/13] x86/apic: Ignore secondary threads if nosmt=force

[Andrew Cooper <andrew.cooper3@citrix.com>]

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On 28/06/2018 23:13, speck for Dave Hansen wrote:
> On 06/20/2018 01:19 PM, speck for Thomas Gleixner wrote:
>> +	/*
>> +	 * If SMT is force disabled and the APIC ID belongs to
>> +	 * a secondary thread, ignore it.
>> +	 */
>> +	if (apic_id_disabled(apicid)) {
>> +		pr_info_once("Ignoring secondary SMT threads\n");
>> +		return -EINVAL;
>> +	}
> Thomas, this boottime-disable stuff just ends up ignoring the
> hyperthread and leaves it alone, right?

Yes

>
> Some Intel folks pointed out a few problems with this.  One is with
> machine checks.  If one thread is booted and has CR4.MCE=1, but the
> other never gets booted and still has CR4.MCE=0, things go boom
> (everything goes to shutdown state) if a machine check happens.
>
> We've traditionally pretended that this does not happen because folks
> don't tend to turn off CPUs they've paid for via things like maxcpus= in
> the real world.
>
> Ashok Raj and Tony Luck were evidently looking at this at some point,
> but it got tricky and decided it wasn't worth the trouble.
>
> It makes me think we should either scrap or recommend against "nosmt=force".
>
> Some relevant SDM language:
>
>> Because the logical processors within a physical package are tightly
>> coupled with respect to shared hardware resources, both logical
>> processors are notified of machine check errors that occur within a
>> given physical processor. If machine-check exceptions are enabled
>> when a fatal error is reported, all the logical processors within a
>> physical package are dispatched to the machine-check exception
>> handler. If machine-check exceptions are disabled, the logical
>> processors enter the shutdown state and assert the IERR# signal. When
>> enabling machine-check exceptions, the MCE flag in control register
>> CR4 should be set for each logical processor.

So what you're saying is that we need to boot all the threads, including
MCE setup etc, then leave them alone (mwait/deep C states?) so they
avoid causing a shutdown?

If so, I've got quite a lot of extra work to do in Xen...

~Andrew